Chocolate cake

1. In a bowl, mix and sift together the dry ingredients, cake flour, unsweetened cocoa powder, baking powder, baking soda and salt.
2. Place the eggs, sugar, vanilla extract, vegetable oil and buttermilk in a large bowl and mix with a hand whisk or electric mixer until smooth.
3. Add the flour mixture and cooled black coffee alternately and in batches, mixing between each addition with an rubber spatula until the mixture is smooth and a little runny.
4. Preheat the oven to 329°F / 165°C and line 3 6 inches/ 15 cm diameter cake tins with baking parchment.
5. Divide the mixture into 3 and divide between the 3 pans (if you only have one or two pans, bake in several batches).
6. Bake for 35 - 40 minutes, until the center is well done. (Check by plunging a toothpick into the center - it should come out clean or with crumbs).
7. Allow the cakes to cool slightly before removing from the pan and cooling completely on a cake rack, upside down.

Oreo Swiss buttercream

1. Prepare my Swiss buttercream recipe. 
2. At the end of the preparation, add the Oreo crumbs, finely blended in a food processor, and mix until smooth.
3. Keep at room temperature until ready to assemble.

Oreo Filling

1. Place all the ingredients in a large bowl, the cold mascarpone cheese, the cold heavy cream, powdered sugar, vanilla extract and the Oreo crumbs.
2. Using an electric mixer, beat at medium speed for 2-3 minutes until smooth and firm.
3. Keep in a cool place until ready to assemble.

Chocolate drip

1. Melt the chocolate chips with the heavy cream in a double boiler or in the microwave until the mixture is liquid and smooth.
2. Leave the chocolate ganache drip to cool at room temperature until it is fluid and less liquid (around 88°F/ 31°C)

Assembly

1. Using a cake leveler or large serrated knife, cut out the bumps in the 3 chocolate cake layers.
2. Place a cake board on a cake decorating turntable, then place a first layer of chocolate cake.
3. Apply a boudin of Swiss meringue buttercream to the outer edges of the first cake layer, then fill the inside with Oreo filling and sprinkle a few crushed Oreo cookie.
4. Cover with the second layer of chocolate cake and repeat the last step, finishing with the final layer of cake.
5. Apply a very thin layer of oreo Swiss meringue buttercream over the entire cake, using an angled spatula to seal in any crumbs, then place your layer cake in the fridge for around 10-15 minutes (to harden the crumb-coating).
6. Then cover the oreo cake with a new, thicker layer of buttercream and smooth it out with icing smoother until you have a smooth outer finish, then place it back in the fridge for a further 20 minutes. 
7. Drizzle the chocolate drip over the top of the cake and use the spatula to bring it to the edges. 
8. Decorate the top of the cake with the remaining buttercream in a piping bag fitted with a decorative tip (in this case tip 1B), then add whole Oreo cookies.
9. Place the chocolate Oreo cake back in the fridge for about 1 hour before serving.