For the cake:
For the frosting:
1. Make the cake: preheat oven to 350F (175C). Line the bottoms of three 6-inch (cm) with parchment and grease the sides.
2. Sift flour, cocoa powder, baking soda, baking powder and salt to a large bowl. Add crushed Oreo and stir well. Set aside.
3. In a large bowl cream together softened butter and sugar until light and fluffy. Add eggs and vanilla extract and beat until incorporated. Gradually add flour mixture and beat until incorporated, then add buttermilk and beat until incorporated. Repeat two times.
4. Evenly divide the batter between three pans. Bake for 20-25 minutes, until a toothpick inserted in the center comes out clean. Let cool for 5-10 minutes in the pan. Release from the pan and allow to cool completely. cut the tops of the cakes to make it flat.
5. Frosting: in a food processor crush the cookies into fine crumbs. In a large bowl beat cream cheese and butter until combined, smooth and creamy, gradually add powdered sugar, vanilla extract and beat until light and fluffy. Fold in crushed Oreo cookies.
6. Assembly: Place one cake layer with flat side down. Spread a layer of frosting, place the second layer of cake on top of the frosting, flat side up. Spread another layer of frosting, then place the third cake on top. Spread evenly the remaining frosting on top and sides of the cake. Refrigerate the cake while making the ganache.
7. Chocolate ganache: Chop the chocolate, place in a large bowl. In a saucepan bring the heavy cream to a simmer. Pour over the chocolate. Let sit for 1-2 minutes. Stir until smooth.
8. Pour the ganache on top and allow to drip. Smooth top with an offset spatula. Pipe the remaining frosting on top of the ganache. And decorate with halved Oreo cookies.