Chocolate Buttermilk Cake

1. In a large bowl, sift together the dry ingredients — cake flour, unsweetened cocoa powder, baking powder, baking soda, and salt.
2. In another bowl, whisk together the eggs, granulated sugar, vanilla extract, and vegetable oil until smooth.
3. In a separate small bowl, mix the buttermilk with the cooled black coffee.
4. Gradually add the dry ingredients and the buttermilk-coffee mixture to the batter in several additions, mixing gently with a spatula after each one until smooth and slightly runny.
5. Preheat the oven to 329°F (165°C). Prepare three 6-inch (15 cm) round cake pans lined with parchment paper. Divide the batter evenly between them (bake in batches if needed).
6. Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.
7. Let the cakes cool slightly in their pans, then remove and cool completely upside down on a wire rack before assembling.

Oreo Swiss Meringue Buttercream

1. Whisk egg whites and granulated sugar in a heatproof bowl set over gently simmering water until the sugar is fully dissolved and the mixture reaches 160°F (71°C).
2. Transfer to a stand mixer fitted with the whisk and beat on high speed for 10–15 minutes, until the meringue is stiff, glossy, and completely cool.
3. Lower the speed and gradually add the softened butter, vanilla, and a pinch of salt; keep mixing until the buttercream comes together and turns smooth.
4. Increase to medium-high speed and whip until thick, fluffy, and silky.
5. Fold in the crushed Oreo cookies, then switch to the paddle attachment and mix on low speed for 1–2 minutes to smooth the texture and remove air bubbles.

Oreo Whipped mascarpone Frosting

1. Place cold mascarpone, cold heavy cream, powdered sugar, vanilla extract, and crushed Oreos in a large mixing bowl.
2. Beat with an electric mixer on medium speed for 2–3 minutes, until thick, creamy, and firm.
3. Refrigerate until ready to assemble the cake.

Chocolate Drip

1. Melt semi-sweet chocolate chips with heavy cream in a microwave or double boiler until smooth and shiny.
2. Let it cool to about 90°F (31–32°C) before pouring, so it’s fluid but not too runny.

Assembling the cake

1. Level the top of each cake with a cake leveler or serrated knife.
2. Place the first cake layer on a cake board or turntable, pipe a ring of Oreo buttercream around the edges, and fill the center with Oreo whipped frosting and crushed Oreo pieces.
3. Repeat with the second layer, then top with the final cake layer.
4. Apply a thin crumb coat using the Oreo buttercream and chill for 10–15 minutes.
5. Add a second, thicker coat and smooth it with an icing smoother. Chill for another 20 minutes.
6. Pour the chocolate drip over the top and gently push it toward the edges to create a drip effect.
7. Pipe decorative swirls of Oreo buttercream (using a 1B piping tip) and finish with whole Oreos on top.
8. Refrigerate the cake for at least 1 hour before serving.