1. Preheat oven to 350 degrees and line the bottom of a 9 inch cake pan with parchment paper and grease the sides.
2. Add the butter and sugars to a large mixer bowl and beat together to cream them until light and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
3. Add the egg and vanilla extract and mix until well combined.
4. In a separate bowl, combine the flour, cornstarch, baking soda and salt.
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. Place a few more chocolate chips into the top of the cake, if desired.
8. Bake for 18-20 minutes or until the edges are slightly golden.
9. Remove from oven and allow to cool completely in the cake pan, then transfer to a serving plate.
1. To make the buttercream, add the butter and shortening 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 water and mix until well combined and smooth.
4. Add the rest of the powdered sugar and mix until smooth. Add more water 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. Cookie cake is best for 3-4 days.
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