COOKIE DOUGH: In medium mixing bowl, whisk together flour, cornstarch, cream of tartar, baking powder, baking soda & salt. Set aside.
In the bowl of a stand mixer (fitted with the paddle attachment), beat the butter on medium speed until smooth & creamy (roughly 2 minutes). Add the sugar & mix on medium until light (about 4 minutes). Use a rubber spatula to scrape the bowl. Add egg whites & vanilla. Mix until fully incorporated.
Add half the flour mixture & mix on low for about 15 seconds. Add the remaining flour mixture & mix until combined. Scrape the bowl & mix an additional 15 seconds. Place half the dough in fridge & leave the rest at room temperature.
GOOEY BUTTER CAKE: Preheat oven to 350 F. In large mixing bowl, mix the cake mix, 2 of the eggs, the melted butter & salt. Stir with a rubber spatula until combined.
In the bowl of a stand mixer (with the paddle attachment), beat the cream cheese until very smooth. Add the 2 remaining eggs & beat until combined. Add the confectioner's sugar & mix until smooth.
Line a 9"x13" pan using parchment paper. Use a rubber spatula to spread the cake mix on the bottom off the pan until you have one even layer. Take chunks of the room temperature sugar cookie dough in your hands & flatten it out into disks. Place the disks on top of the cake layer, elating a 1 inch border around the edges. Continue until you use all of the room temperature cookie dough.
Pour the cream cheese mixture over the pan & spread evenly. Tap the pan on the counter to release any bubbles.
Bake for 35-45 minutes, until the edges are golden brown & a delicate crust has formed on the top. Allow to cool for 5 minutes. Sprinkle the rainbow sprinkles all over the cake.
Break the refrigerated cookie dough into pea size pieces & sprinkle the cookie dough over the cake. Let cool another 15 minutes, cut into bars, serve & enjoy!