★★★★★

Soft and Chewy Frosted Rainbow M&M Cookie Bars Recipe

Soft and chewy M&M Cookie bars topped with a sweet rainbow buttercream frosting. These frosted Rainbow M&M Cookie Bars are dreamy and delicious!

Squares of chocolate chip cookie bars on a white plate. Frosted with a swirl of rainbow frosting.

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I love frosting.

Preferably a good buttercream frosting.

And this one has rainbow colors. 

It also sits on top of a delicious, soft and chewy, M&M Cookie bar.

What’s not to love?

I happen to love a lot of frosting.

Give me a giant slice of cake, from the corner, with extra frosting please.

While some of my family scrapes the extra frosting off their cake, I pile it on to my slice.

How to Make Rainbow Frosting

Plate of square cute rainbow frosted chocolate chip cookies bars with a plate of plain (non frosted) bars in the back ground.

Start by making your basic white buttercream frosting.

I make a larger batch so that I can put a decent sized “rainbow” on each individual cookie bar (or cupcake!). You can cut the batch in half if you think you prefer less frosting.

Begin by beating softened butter for about 3 minutes using an electric mixer with whisk attachment. Scrape down the sides of the bowl as needed, until you see the butter turn pale in color, and smooth.

Add powdered sugar, milk, and pure vanilla extract.

Beat an additional 3-5 minutes until light and fluffy.

Now decide how many colors in your rainbow!

PRO TIP: Use Americolor gel colors for more vibrant colors that require less drops. Each of my colors was ONE drop of gel. 

4 clear decorator bag with the colored frosting...blue, yellow, pink, purple.

I chose four pastel colors for my spring cookies!

Separate your frosting into small bowls (for each color). Add one drop of gel coloring to each bowl, stir until no white frosting is seen. Add more color if desired.

Spoon frosting into small 12-inch disposable frosting bags. Snip off the tips.

Carefully slide all four bags into a LARGER frosting bag. I used an 18-inch bag with star tip attached.

Pipe frosting onto cooled cookie bars!

This is also very pretty piped onto cupcakes!

Rose swirl of rainbow frosting on a square of parchment paper with frosting bag and tip next to it. M&M's in background with cookie bars.

Can I freeze frosting?

Yes!

Frosting will stay good in the refrigerator for 4-5 days.

However, should you choose to freeze frosting, it can keep in an airtight container for up to 3 months in the freezer.

Thaw frosting overnight in the refrigerator.

You may need to beat it again with your mixer to get it light and fluffy, and you may need to add 1-2 TBSP of milk too.

White scalloped plate with squares of rainbow frosted cookie bars.

What can I do with leftover frosting?

Help! This was too much frosting for me.

While I personally don’t understand that phrase, I do know that some people don’t like as much frosting as I do.

In addition to freezing frosting, you can also pipe it onto sugar cookies for a delicious treat.

Or add frosting to these Bakery style brownies.

Still not sure how to use your leftover frosting?

Make a batch of Buttercream Truffles. That’s right, sweet candy made from your LEFTOVERS. Dream come true!

Chocolate Chip cookie bars with rainbow swirled frosting on a white plate with yellow napkin underneath.

More Rainbow Desserts

My family loves these soft and chewy Rainbow Cookies. They’re basically M&M Chocolate Chip, with a delicious chewy texture!

These Rainbow Chip Brownies are made using the best homemade buttermilk brownie recipe! Move over Cosmic Brownies from Little Debbie…these are too delicious to compare!

You’ll love these decadent Rainbow Fudge Cookie Bars. Chocolate Chip Cookie bars topped with a sweet fudge and rainbow chips!

Soft and chewy M&M Cookie bars topped with a sweet rainbow buttercream frosting. These Rainbow M&M Cookie Bars are dreamy and delicious!

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More Easy Desserts:

Yield: 24 bars

Rainbow M&M'S Cookie Bars

Prep Time 15 minutes
Total Time 15 minutes

Rainbow Cookie Bars: delicious chocolate chip cookie bars topped with a rainbow buttercream (tutorial)

Ingredients

For the Frosting:

  • 1 cup unsalted Butter, softened
  • 5 1/2 cups powdered sugar
  • 1/4 cup milk
  • 2 tsp vanilla extract
  • gel food coloring

Instructions

  1. For the cookies, bake and cool one batch of chocolate chip cookie bars.
  2. Start by making your basic white buttercream frosting.
  3. I make a larger batch so that I can put a decent sized "rainbow" on each individual cookie bar (or cupcake!). You can cut the batch in half if you think you prefer less frosting.
  4. Start by beating softened butter for about 3 minutes using an electric mixer with whisk attachment. Scrape down the sides of the bowl as needed, until you see the butter turn pale in color, and smooth.
  5. Add powdered sugar, milk, and pure vanilla extract.
  6. Beat an additional 3-5 minutes until light and fluffy.
  7. Now decide how many colors in your rainbow!
  8. PRO TIP: Use Americolor gel colors for more vibrant colors that require less drops. Each of my colors was ONE drop of gel. 
  9. I chose four pastel colors for my spring cookies!
  10. Separate your frosting into small bowls (for each color). Add one drop of gel coloring to each bowl, stir until no white frosting is seen. Add more color if desired.
  11. Spoon frosting into small 12-inch disposable frosting bags. Snip off the tips.
  12. Carefully slide all four bags into a LARGER frosting bag. I used an 18-inch bag with star tip attached.
  13. Pipe frosting onto cooled cookie bars!

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Nutrition Information:

Yield:

24

Serving Size:

1

Amount Per Serving: Calories: 174Total Fat: 8gSaturated Fat: 5gTrans Fat: 0gUnsaturated Fat: 3gCholesterol: 21mgSodium: 7mgCarbohydrates: 26gFiber: 0gSugar: 25gProtein: 0g

*Nutrition facts are an estimate and not guaranteed to be accurate. Please see a registered dietitian for special diet advice.

Did you make this recipe?

Please leave a comment on the blog or share a photo on Instagram

Recipe originally published April 10, 2012. Photos updated April 2019.

Aimee

About Aimee

Aimee is a dessert lover (specifically rice krispie treats), workout enthusiast, self-taught foodie, and recipe creator.

Learn more about Aimee.

Affiliate Disclosure:**There may be affiliate links in this post! By clicking on them, or purchasing recommended items I may receive a small compensation. However, I only recommend products I love! Thank you for supporting Shugary Sweets! See my disclosure policy for more info**

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Posted on April 29, 2019

Comments & Reviews

  1. We love m&m cookie bars & the addition of the cute frosting on top, would make them even better! My girls are all about rainbows!

  2. These look perfectly decadent! I will have to make these for my daughter’s birthday party, she will love them!

  3. These cookie bars were a hit around here! My daughters made them and frosted according to your tips. Cutest treat ever!

  4. I need help…i baked your recipe for blondies the other day…they were still raw…i rebaked them for another 20 min and still so soft u cant pick them up…i followed recipe exactly and i still cannot seem to find a good bkondie that is “chewy” without being sloppy raw like…

    1. Hmm. Not sure what to tell you. I’ve been making that blondies recipe for years with no issue 🙁

  5. These are so colorful and I love the recipe for the basic cookie bar!You are a party fave this week at the {what’s shakin’ link party} … I’d love for you to come on over, link up and grab a button!

  6. These look so delicious! We’re so glad that you linked up to our “Strut Your Stuff Saturday.” We hope you’ll be back soon. -The Sisters

  7. Oh my goodness these look so yummy! I followed you over from Chef-in-Training and would like to invite you to share this, and any other posts, on my link party at www.michellestastycreations.blogspot.com. Have a great weekend.

    Hugs,
    Michelle

  8. That looks terrific. I know my family would love this. Come visit us. We have some fantastic things to share this week.

  9. What a fun spring time treat. I will definitely have to make these. Love your blog and I am your newest follower. xo

  10. these are too cute! i’m adding them to my to-bake list. i love all those colors! awesome recipe. 🙂

  11. Those are so pretty! Love what you’ve done with the frosting … I am not the biggest, best froster, but I think I need to try that technique. Sometime.

  12. They look awesome, your frosting turned out so well! I just love the technique – such an impressive result and really not too difficult 🙂

  13. Wonderful recipe! I thik I’ll make this for Hubby tonight to take to twork. His co-workers are chocolate chip M&M fans!

  14. I love that frosting! I have never seen one with meringue powder and butter. It sounds SO good. I will be trying it for sure!

  15. I love the rainbow frosting! It’s so pretty and looks so intricate. I don’t have the patience to do that but I really want to now seeing yours!

  16. I have a couple in our house that scrape the yummy frosting off. But you are right…more for us:-) Love the fun rainbow icing!!!!

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