Preheat oven to 350F (175C) and prepare a 10" round pan by lining the bottom with parchment paper or by lightly greasing. Set aside.
combine softened butter and sugars in a large bowl and use an electric mixer to beat well until light and fluffy.
¾ cup unsalted butter, ¾ cup light or dark brown sugar, ⅔ cup sugar
Add eggs, and egg yolk one at a time, stirring well after each addition.
1 egg + 1 egg yolk
Stir in vanilla extract.
1 teaspoon vanilla extract
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
2 cups all-purpose, 1 ½ teaspoon corn starch, ½ teaspoon baking powder, ¾ teaspoon salt
Gradually stir dry ingredients into wet until completely combined. Stir in chocolate chips and sprinkles.
1 cup semisweet chocolate chips, 2 Tablespoons sprinkles
Spread batter evenly into prepared pan. Bake on 350F (175C) for 30 minutes or until edges just begin to turn golden brown.
Prepare chocolate frosting while the cookie cake cools.
Chocolate Frosting
Cream butter with an electric mixer.
½ cup 1 stick salted butter, softened to room temperature
Gradually add sugar until completely combined
1 ½ cups powdered sugar
Stir in cocoa powder and then vanilla extract. Pause to scrape down the sides of the bowl with a spatula.
¼ cup cocoa powder, ½ teaspoon vanilla extract
Add milk (start with just 1 teaspoon, if consistency is still too stiff add another) and beat well
1-2 teaspoons milk
Pipe icing around the rim of your cooled cookie cake -- I used a piping bag fitted with a Wilton 2D tip, but in a pinch you can simply transfer your frosting to a ziploc bag, snip off a small corner, and pipe that way.
Slice and enjoy!
Video
Notes
Pan
A 9" springform pan may be used instead, cake will need to bake approximately 35 minutes.