Step 1

Preheat the oven to 350°F (175°C). Grease a 9-inch cake pan or pie dish and line it with parchment paper.

Step 2

Make the cookie dough: In a medium bowl, whisk the flour, cornstarch, baking soda, baking powder, and salt. Set aside.

Step 3

In a large mixing bowl, beat the butter, brown sugar, and granulated sugar with an electric mixer on medium speed until light and fluffy, about 2-3 minutes. Reduce the speed to low and blend in the egg, egg yolk, and vanilla until smooth.

Step 4

Gently stir the dry ingredients into the wet ingredients by hand with a spatula until about halfway combined. Add 1 1/4 cups chocolate chips and mix until just combined.

Step 5

Press the dough evenly into the prepared pan. Sprinkle the remaining chocolate chips on top.

Step 6

Bake for 20-25 minutes, until the edges are lightly browned. The center should still be slightly soft. Let the cookie cake cool completely in the pan.

Step 7

Make the frosting: In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar, cocoa powder, 2 tablespoons milk, vanilla, and salt, and beat until smooth and fluffy. Add more milk, as needed.

Step 8

When the cookie cake is cool, remove it from the pan and decorate with frosting as desired. Add sprinkles for a festive touch, serve, and enjoy!