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 and pecans. The cookie dough will be thick.
7. Spread half of the dough evenly into the prepared cake pan. Spread marshmallow fluff evenly on top of cookie dough, leaving a little space around the edge.
8. Drop spoonfuls of the remaining cookie dough over the marshmallow fluff and carefully flatten into an even layer.
9. Bake for 18-20 minutes. The center may still look a little undercooked but it will firm up and continue cooking as it cools.
10. Remove from oven and allow to cool completely in the cake pan, then transfer to a serving plate.

Make the buttercream and decorate

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 vanilla extract and marshmallow fluff and mix until well combined and smooth.
4. Add the rest of the powdered sugar and mix until well combined and smooth.
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. Decorate with some chopped pecans, chocolate chips and mini marshmallows, if desired.
6. Store cookie cake in an airtight container. Because of the buttercream, it should be refrigerated after about 48 hours. Cookie cake is best if eaten within 3-4 days.