Cookie Cake:

1. Preheat oven to 350°F (180°C). Prepare a 9-inch cake pan with parchment paper in the bottom and baking spray on the sides. Set aside.
2. Combine the flour, cornstarch, baking soda and salt in a medium sized bowl and set aside.
3. Cream the butter and sugars together in a large mixer bowl on medium speed until light in color and fluffy, 2-3 minutes. You should be able to see the change in color and texture and know it’s ready. Do not skimp on the creaming time.
4. Add the egg and vanilla extract and mix until well combined.
5. Add the dry ingredients to the butter mixture and mix until the dough is just well combined. Do not over mix. Once it’s well combined, use a rubber spatula to help it come together to form a more cohesive ball.
6. Fold in the chocolate chips. The cookie dough will be thick.
7. Press the cookie dough evenly into the cake pan. Press a few more chocolate chips into the top of the cake, if desired.
8. Bake for 18-20 minutes, or until the edges just begin to turn golden.
9. Remove from oven and allow to cool completely in the cake pan, then transfer to a serving plate.

Chocolate Buttercream:

1. To make the buttercream, add the butter to a large mixer bowl and beat until well combined and smooth.
2. Add about half of the powdered sugar and mix until well combined and smooth.
3. Add the cocoa powder, vanilla extract and a tablespoon of cream and mix until well combined and smooth.
4. Add the rest of the powdered sugar and mix until well combined and smooth. Add more cream to get the right consistency.
5. Add the buttercream to a piping bag fitted with a piping tip (I used Ateco 844) and pipe swirls of frosting around the outer edge of the cake.
6. Store cookie cake in an airtight container. You can store it at room temperature for about 24 hours, then store in the fridge. Without the buttercream, it can remain at room temperature. I recommend serving at room temperature, since the cookie will firm up when cold. Cookie cake is best eaten within 4-5 days.