Make the 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, baking soda and salt in a medium sized bowl and set aside.
3. Cream the butter, peanut butter and sugars together in a large mixer bowl on medium speed until light in color and fluffy, about 2-3 minutes. Scrape down the sides of the bowl a few times to be sure all is well incorporated. You should be able to see the change in color and texture happen and know it’s ready.
4. Add the egg and vanilla extract and mix until well combined.
5. Add the dry ingredients and mix until the dough is just combined. Do not over mix.
6. Stir in the chopped Reese’s and chocolate chips just until evenly distributed.
7. Press the cookie dough evenly into the cake pan. Press a few more chocolate chips and/or chopped Reese’s into the top of the cookie cake, if desired.
8. Bake for 18-22 minutes, or until the edges begin to turn golden and the center just looks set.
9. Remove from oven and allow to cool completely in the cake pan, then transfer to a serving plate. It it’s fully cool, you should be able to flip it upside down onto a cooling rack, place another cooling rack on top (which is actually the bottom of the cookie) and then flip it back over so that it’s right side up.

Make the buttercream

1. To make the buttercream, add the butter to a large mixing bowl and beat until smooth.
2. Add 1 cup of powdered sugar and mix until combined.
3. Add the cocoa powder and vanilla extract and mix until well combined. Add a tablespoon of cream and mix until well combined.
4. Add the remaining powdered sugar and mix until well combined and smooth. Add additional cream, if needed.
5. Pipe the buttercream around the edges of the cookie cake, adding half a Reese’s between each swirl of buttercream. Store in an airtight container until ready to serve. Store at room temperature for up to 2 days, then refrigerate because of the buttercream. Serve at room temperature. Best if eaten within 4-5 days.