Step 1

Preheat the oven to 350°F. Line the bottom of a 12-inch round cake pan** or oven-safe skillet with parchment paper in a criss-cross pattern.

Step 2

In a large mixing bowl combine flour, cornstarch, baking soda, and salt. Set aside.

Step 3

In a medium bowl combine melted butter, brown sugar, granulated sugar, eggs, and vanilla.

Step 4

Add the sugar mixture to the flour mixture and stir until combined. Fold in ¾ cup of chocolate chips. Press dough into the prepared pan and spread the remaining chips on top.

Step 5

Bake for 18-20 minutes, or until the edges are golden brown.

Step 6

Let the cookie cool completely before frosting (about an hour). While the cookie is cooling, prepare the frostings.

Step 7

To the bowl of a stand mixer with the paddle attachment, add the brown sugar and butter. Beat on medium speed until smooth and creamy.

Step 8

Mix in flour, vanilla extract, and salt until fully combined.

Step 9

With the mixer on low speed, add milk, 1 tablespoon at a time until fully incorporated.

Step 10

Remove the bowl from the mixer. Fold in chocolate chips.

Step 11

To the bowl of a stand mixer fitted with the paddle attachment, add the butter, sugar, salt, and vanilla extract. Start mixing on low speed until the sugar is incorporated with the butter. Increase the mixing speed to high and beat until smooth and fluffy, about 2 minutes

Step 12

Add milk, one teaspoon at a time, until the frosting reaches your desired consistency. You will have about 3 cups of frosting.

Step 13

Divide the frosting equally in half, placing one half in a separate bowl. You will use one-half of the frosting to make the chocolate buttercream.

Step 14

To the half amount of American buttercream, add the cocoa powder. Mix until combined.

Step 15

To frost, cut the cooled cookie cake into 12 equal wedges. Pipe the first frosting onto every third wedge. (I used a #32 tip) Repeat with the remaining two frostings.