Bigger is better, especially when it comes to cake. This huge cookie-cake hybrid is packed with rainbow sprinkles and makes a big, colorful splash at any celebration.
To make buttercream:
In a stand mixer fitted with a paddle attachment, cream butter on medium high until fluffy. Turn speed to low and gradually add sugar. Once combined, turn back up to medium high and continue to beat until light and fluffy. Add vanilla, then cream. Prepare a pastry bag with piping tip and fill with the frosting.
To make cookie:
Preheat oven to 350°F. Spray a 9-inch pie dish or cake pan with nonstick spray. Set aside.
In a stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Get it nice and smooth, then add the sugar and increase to medium speed and beat until fluffy and light in color. Add in egg, egg yolk, and vanilla. Scrape down the sides as needed.
In a separate medium bowl, whisk together the flour, salt, baking powder, baking soda, and cornstarch. With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 separate additions. The dough will be quite thick. Once combined, gently fold in the white chocolate chips and 1/2 cup sprinkles using a rubber spatula.
Press the cookie dough evenly into the prepared pan. At this point you may sprinkle additional sprinkles on the top of cake. Bake for 25-30 minutes or until very lightly browned on top. You want the cookie cake to be extra soft in the center, so be careful not to over bake.
Allow the cake to cool completely on a wire rack at room temperature before frosting. The center will sink slightly.
Once fully cooled, frost the cookie cake with buttercream and garnish with even more sprinkles.