Preheat oven to 350 degrees F. Line a 13x9 baking dish with parchment paper (or foil). Set aside.
In a large bowl, combine butter and sugars until well blended. Add in eggs, egg yolk, and vanilla extract. Mix until combined.
Add in flour, baking soda, cornstarch, and kosher salt just until combined. Fold in chocolate chips and M&M's.
Press cookie dough into baking dish.
Bake 35 minutes until lightly browned. Remove and cool completely.
Store in airtight container for up to 3 days.
Notes
One extra egg yolk. You’re wondering if it’s worth it. Should you skip it? Should you add the whole egg? Why just an extra egg yolk? Well, the extra egg yolk is just enough egg in this recipe to help these bars stay chewy and soft. Yes, you can toss in a whole egg if you prefer…but you came here for the PERFECT Chocolate Chip Cookie Bar…and this extra egg yolk is why it’s the best!
When spreading the cookie dough into the baking dish, dip your fingers in flour first. This keeps the dough from sticking to YOU!
Remove from oven when lightly browned. Don’t overcook. These bars will continue to bake for a few minutes in the dish.
I like my cookie bars with extra chocolate chips. You’ll see the fudgy center of my cookies bars, that’s because I always throw in an extra 1/4-1/2 cup chocolate chips. Add to taste!
Store cookie bars in airtight container for up to 3 days.