Snickerdoodle Cookie Bars

Preheat the oven to 350 degrees F. Line an 8x8 square pan with parchment paper.
Mix together the dry ingredients in a large mixing bowl - all purpose flour, cornstarch, cream of tartar, baking powder, ground cinnamon, and kosher salt. Give it a good whisk and set aside.
Add room temperature butter to the bowl of a stand mixer fitted with the paddle attachment (you can also use an electric hand mixer here). Mix on low speed until combined, about 1 minute. Then add brown sugar and sugar and mix for an additional 2 minutes on medium speed. Scrape down the sides of the bowl and add vanilla extract, whole egg, and egg yolk. Mix for an additional 1-2 minutes, until everything is well incorporated.
Add the dry ingredients to the bowl and mix for 30 seconds to 1 minute, until there are no streaks of flour and a thick, soft dough forms. Scoop the cookie dough into the prepared pan and smooth into an even layer (it's help to use an offset spatula).
Then make the cinnamon topping. Mix together flour, brown sugar, and cinnamon in a bowl. Pour in melted butter and mix until a sandy mixture forms. Sprinkle on top of the cookie dough in an even layer.
Bake in the preheated oven for 30-34 minutes, until the top of the cookie bars are golden brown and the edges are set. Allow the bars to cool in the pan for 10 minutes, then remove onto a wire rack to cool completely prior to cutting into them!