Preheat oven to 350 degrees F. Line a 13x9 baking dis with parchment paper. Set aside.
In a large mixing bowl, beat together butter and sugar for about 2 minutes. Add in eggs and beat until light and fluffy.
Add in flour, cream of tartar, baking soda, and salt. Beat just until dough comes together.
Pour cookie dough into bottom of prepared baking dish. Using your hands (or the bottom of a cup), press the dough evenly into the bottom of the dish.
In a small bowl, mix together the topping of cinnamon and sugar. Sprinkle over the top of the cookie dough (it may seem like a lot, but it's worth it)!
Bake cookie bars for 22-26 minutes. Remove from oven and cool completely in pan. Cut into bars and enjoy.
Notes
Store cookie bars in airtight container for up to one week.
Freeze cookie bars in freezer safe container for up to three months.
Why use cream of tartar? Cream of tartar gives snickerdoodles their unique flavor and chewy texture. If you don't have any on hand, swap it out for 2 teaspoons baking powder instead.
Make it over the top by adding a drizzle of vanilla icing or melted white chocolate!