• 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 egg, egg yolk, and vanilla extract and stir well.
  1 egg + 1 egg yolk, 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

• Beat butter with an electric mixer until smooth and creamy.
  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 a splash, if consistency is still too stiff add more as needed until you have a smooth creamy consistency) and beat well
  1-2 Tablespoons 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!