Make the Cookie Cake

1. Preheat oven to 350°F (180°C) and line the bottom of a 9 inch cake pan with parchment paper and grease the sides.
2. In a medium sized bowl, combine the flour, cocoa powder, baking soda and salt. Set aside.
3. Add the butter and sugars to a large mixer bowl and beat together until light in color and fluffy in texture, about 2-3 minutes. 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 beat just until combined. Use your spatula to gently finish combining everything to keep from over mixing the cookie dough.
6. Fold in the chocolate chips. The cookie dough will be thick.
7. Spread the dough evenly into the prepared cake pan. Press a few more chocolate chips into the top of the cookie cake, if desired.
8. Bake for 18-20 minutes or until it looks baked, but still wiggles a little when you jiggle it.
9. Remove from oven and allow to cool completely in the cake pan, then transfer to a serving plate.

Make the 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 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. Serve.
6. Store cookie cake in an airtight container. Cookie cake is best if eaten within 3-4 days.