FOR THE COOKIE DOUGH LAYER: 3 cups all-purpose flour 1 tablespoon cornstarch 1 1/2 teaspoons cream of tartar 3/4 teaspoon baking soda 3/4 teaspoon salt 1 1/2 sticks unsalted butter, room temperature 1 cup plus 2 tablespoons sugar 1/3 cup pasteurized egg whites, room temperature 1 1/2 teaspoons vanilla extract
FOR THE CAKE: 1 box yellow cake mix 4 whole eggs, at room temperature (divided) 1 stick unsalted butter, melted 1 teaspoon salt 1 (8 oz) package cream cheese, softened 1 (1 lb) package confectioners’ sugar, plus more for topping 1/2 cup rainbow sprinkles
TOOLS YOU'LL NEED: 1 microwave-safe bowl 1 instant-read thermometer 1 medium bowl 1 stand mixer or hand mixer 1 mixing bowl 1 rubber spatula 1 9"x13" baking pan
// Heat-treat the flour: Place the flour in a microwave-safe bowl. Microwave on high for 30 seconds at a time, stirring after each interval. Stir well to make sure none of the flour burns (microwaves have those tricky hot spots). Use an instantread thermometer to test the flour in several places to make sure it has reached 165°F throughout. If you get a lower reading in one area, just stir and heat for an additional 30 seconds until it’s all ready! Let cool completely.
// Make the sugar cookie dough: In a medium bowl, whisk together 2 1/2 cups of the heat-treated flour, cornstarch, cream of tartar, baking powder, baking soda and salt. Set aside. // In the bowl of a stand mixer with a paddle attachment, beat the butter on medium speed until smooth and creamy, about 2 minutes. Add the sugar and mix on medium until fluffy; about 4 minutes. Use a rubber spatula to scrape the bowl. Add the egg whites and vanilla and mix until fully incorporated, about 2 minutes.
// Add half of the flour mixture and mix on low until the powdery texture of the flour disappears, about 15 seconds. Immediately add the rest of the flour mixture and mix until combined. Scape the bowl and mix for 15 more seconds. // Remove the bowl from the mixer and place about 1 1/2 cups of the dough in the refrigerator and leave the rest at room temperature while you make the cake. // Preheat the oven to 350° F.
// Make the cake: In a large bowl, mix the cake mix, 2 of the eggs, the melted butter, and salt. Stir with a rubber spatula until combined. // In the bowl of a stand mixer with a paddle attachment, beat the cream cheese until very smooth. Add the 2 remaining eggs and beat until combined. Add the confectioners’ sugar and mix until smooth // Assemble the cake. Line a 9” x 13” pan with parchment paper. If you plan to serve the bars straight from the pan, skip the parchment and lightly coat the pan with cooking spray. Use a rubber spatula to spread the cake mix on the bottom of the pan until you have one even layer. // Take chunks of the room-temperature sugar cookie dough in your hands and flatten it out into disks. Place on top of the cake layer, leaving a 1-inch border around the edges (the dough will spread as it bakes). Continue until you use all of the room-temperature cookie dough and you have almost completely covered the cake mix layer. // Pour the cream cheese mixture over the pan and spread evenly. Tap the pan on the counter to release air bubbles and create an even layer. // Bake for 35 to 45 minutes, until the edges are golden brown and a delicate crust as formed on the top. The middle should still be wobbly when shaken. Remove from the oven and allow to cool for 5 minutes. Sprinkle the rainbow sprinkles all over the top of the cake. // Meanwhile, remove the reserved cookie dough from the refrigerator. Use your hands to break the dough into pea-size pieces and sprinkle the cookie dough pieces over the cake. Let cool for 15 more minutes. Liberally dust with confectioners’ sugar just before serving. Cut into bars and serve warm. Store the cake covered in the refrigerate or for up to 5 days.
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