1/2 cup (115g) unsalted butter, softened to room temperature
3/4 cup (150g) granulated sugar （我用100g）
1 large egg + 1 egg yolk
2 teaspoons vanilla extract （没放）
1 and 1/2 cups (190g) all-purpose flour
1/4 teaspoon salt
1 teaspoon baking powder
1/2 teaspoon baking soda
1 and 1/2 teaspoons cornstarch
2/3 cup (120g) white chocolate chips （只放80g）
1/2 cup (80g) rainbow sprinkles （只放30g）
Preheat oven to 350°F (177°C). Spray a 9-inch pie dish or cake pan with nonstick spray. Set aside.
Make the cake: In a large bowl using a hand-held mixer or 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 on medium speed until fluffy and light in color. Beat in egg, egg yolk, and vanilla. Scrape down the sides as needed.
In a 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 different parts. The dough is quite thick. Once combined, gently fold in the white chocolate chips and 1/2 cup sprinkles using a rubber spatula (or by mixing on low speed).
Press the cookie dough evenly into the prepared pan. 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 careful not to overbake. Allow the cake to cool completely on a wire rack at room temperature before frosting. The center will slightly sink - that's normal.