To make the cookie cake

1. Preheat oven to 350 degrees. Place parchment paper in the bottom of a 9 inch cake pan and grease the sides.
2. Cream butter and sugars together until light and fluffy, about 3-4 minutes.
3. Mix in egg and vanilla extract.
4. Combine dry ingredients in a another bowl, then add them to the butter mixture and mix until well combined. Dough will be thick.
5. Stir in chocolate chips.
6. Press the dough evenly into the cake pan.
7. Bake for 21-24 minutes or until the edges are slightly golden. The center will look undercooked, but it will continue cooking and firm up as it cools.
8. Remove from oven and set aside to cool completely in the cake pan. It may still look a little undercooked until completely cool and firm. Once cooled, the center should be firm and it’ll be easier to remove it from the pan without it falling apart.

While the cookie cake cools, make the brownie

1. Preheat oven to 350 degrees. Place parchment paper in the bottom of a 9 inch cake pan and grease the sides.
2. To make the brownie, add the butter and chocolate to a heat-proof bowl (I use my mixer bowl) and set it over a pot of simmering water. Melt the butter and chocolate, stirring until smooth. Set aside to cool a bit.
3. Whisk together the sugar, vanilla extract and eggs in a medium bowl until well combined.
4. Combine the dry ingredients in another medium bowl.
5. Whisk the cooled chocolate mixture into the egg mixture and combine.
6. Gently stir the flour mixture into the chocolate mixture until well combined.
7. Pour the batter evenly into the prepared pan and bake for 35-38 minutes, or until a toothpick inserted comes out with a few sticky crumbs. You don’t want to over bake it.
8. Remove brownie from oven and set aside to cool completely in the pan, then remove from the pan.

While the brownie cools, make the Oreo cake

1. Preheat oven to 350 degrees. Place parchment paper in the bottom of a 9 inch cake pan and grease the sides.
2. Whisk together flour, sugar baking powder and salt in a large mixing bowl.
3. Whisk together egg whites, sour cream, milk and vanilla extract in a medium bowl, then add to the flour mixture and mix on medium speed just until smooth.
4. Add melted butter and mix until combined.
5. Stir in the Oreo crumbs and chopped Oreos.
6. Pour batter evenly into prepared cake pan and bake for 34-36 minutes, or until a toothpick inserted in the center comes out with a few crumbs.
7. Remove cake from oven and allow to cool for 2-3 minutes, then remove from pan to a cooling rack to finish cooling.

When the cake layers are ready, make the frosting

1. Beat the butter and shortening together until smooth.
2. Slowly add about half of the powdered sugar, mixing until smooth between each addition of powdered sugar.
3. Add the vanilla extract, cocoa powder and 4 tablespoons of water or milk to the frosting and mix until smooth.
4. Slowly add the remaining powdered sugar, mixing until smooth between each addition of powdered sugar.
5. Add additional water to the frosting as needed to get a smooth consistency.

Put it all together

1. Use a large serrated knife to remove the top edges from the cookie cake so that it’s flat and remove the dome from the top of the oreo cake. If the top of the brownie isn’t flat, trim it so that it is. See my tips on how to level a cake.
2. Place the cookie cake on a serving plate or cardboard cake circle and top with about 1 cup of frosting. Smooth frosting into an even layer.
3. Add about half of the additional crushed Oreos on top of the frosting and press into the frosting.
4. Add the Oreo cake and top with about 1 cup of frosting. Smooth frosting into an even layer.
5. Add remaining half of the additional crushed Oreos on top of the frosting and press into the frosting.
6. Add the brownie to the top of the cake.
7. Add a crumb coat and frost the outside of the cake. For help with getting a smooth cake, refer to my tutorial on how to frost a smooth cake with buttercream.
8. Pipe swirls around the top edge of the cake. I use Ateco tip 844, but Wilton 2D or 1M would work as well.
9. To make the chocolate ganache, add the additional chocolate chips to a heat proof bowl (see my tips on making chocolate ganache).
10. Microwave the heavy whipping cream until boiling, then pour over the chocolate chips. Cover the bowl with clear wrap for about 5 minutes, then whisk until smooth.
11. Use a spoon to drizzle the chocolate ganache down the sides of the cake. See my tips on making a chocolate drip cake.
12. Pour remaining ganache onto the top of the cake and spread into an even layer in the middle of the piped swirls.
13. Finish the cake off with chocolate chip cookies and Oreos that have been cut in half.