1. Make the Chocolate Cake Layers:

Preheat oven to 350°F (176°C). Grease three 6-inch round baking pans, dust with cocoa powder, and line bottoms with parchment paper.

Place all dry cake ingredients (flour, sugar, cocoas, baking soda, baking powder, salt, sifted Oreo crumbs) into the bowl of a stand mixer fitted with the paddle attachment. Stir briefly to combine.

In a medium bowl, whisk all wet cake ingredients (buttermilk, oil, eggs, vanilla). Slowly pour the hot water/coffee into the wet ingredients while whisking constantly to temper the eggs slightly.

With the mixer on low speed, gradually add the wet ingredients to the dry ingredients. Mix on medium speed for 2-3 minutes until well combined and smooth. Batter will be thin.

Pour batter evenly into the three prepared pans (use a kitchen scale for precision if desired).

Bake for 30-35 minutes, or until a cake tester inserted into the center comes out mostly clean.

Cool cakes in pans for 10 minutes, then turn out onto wire racks, remove parchment, and cool completely.

Once cool, carefully cut each cake layer in half horizontally using a long serrated knife to get 6 thin layers.

2. Make the Swiss Meringue Buttercream (SMBC):

Place egg whites and granulated sugar into the clean, heatproof bowl of your stand mixer. Whisk until combined.

Place the bowl over a saucepan containing a few inches of simmering water (a double boiler setup). Ensure the bottom of the bowl doesn’t touch the water.

Whisk constantly until the mixture is warm, no longer grainy to the touch (when rubbed between fingers), and ideally reaches 160°F (71°C) on a candy thermometer (this ensures egg safety), approximately 3-5 minutes.

Carefully transfer the bowl to your stand mixer fitted with the whisk attachment. Add cream of tartar and salt now, if using.

Beat on medium-high speed until the meringue is stiff, glossy, and the bottom of the bowl is completely cool to the touch (this can take 5-10 minutes or more).

Switch to the paddle attachment. With the mixer on medium-low speed, add the room temperature, cubed butter, one piece at a time, mixing until incorporated. The mixture will likely curdle/separate – keep beating!

Once all butter is added, increase speed to medium-high and beat until the SMBC becomes silky smooth (this can take several minutes). Beat in the vanilla extract (clear, if using).

Remove about 1 cup of the plain white buttercream and set aside for piping later.

Add the 1 cup of sifted Oreo cookie crumbs to the remainder of the buttercream in the bowl. Whip on low, then medium speed until combined and smooth.

3. Assemble the Cake:

Place one thin cake layer on a cake stand or serving dish.

Top with about ⅔ cup of the Oreo buttercream and spread evenly. Sprinkle generously with some of the 10 chopped Oreos.

Repeat with the next four cake layers (cake, Oreo buttercream, chopped Oreos).

Place the final (sixth) cake layer on top.

Apply a thin “crumb coat” of the Oreo buttercream all over the cake (top and sides) to trap loose crumbs.

Chill the crumb-coated cake for 20-30 minutes until the frosting is firm.

4. Final Frosting:

Frost the outside of the cake smoothly with the remaining Oreo buttercream using an offset spatula and bench scraper for sharp edges and a smooth finish.

Chill the frosted cake again for at least 30 minutes until firm. This helps the ganache drip set nicely.

5. Make and Apply Ganache:

Add heavy cream to a medium microwave-safe bowl. Add black color gel now if using, and stir. Add the chopped dark chocolate.

Microwave in 10-second intervals at 50% power, stirring well after each, until the chocolate is mostly melted. Let sit for a minute, then stir until completely smooth and shiny. Do not overheat.

Let the ganache cool slightly until it’s thickened but still pourable (test drip consistency on a cool plate or the side of the bowl).

Using a small spoon or a squeeze bottle, carefully apply dollops of ganache around the top edge of the chilled cake, allowing some to drip down the sides naturally.

Fill in the center top of the cake with more ganache and spread evenly with a small offset spatula. Allow the ganache to set for a few minutes (can briefly chill).

6. Final Decoration:

Fit a piping bag with a large star tip (like #1M or #869). Fill with the plain white buttercream that was set aside earlier.

Pipe decorative rosette dollops or swirls around the top edge of the cake, on top of the set ganache.

Place a mini Oreo in the center of each dollop, if desired.

7. Chill and Serve:

Refrigerate the finished cake until ready to serve (at least 30 mins for ganache to fully set).

For best texture, let the cake sit at room temperature for 20-30 minutes before slicing. Slice with a tall, sharp knife dipped in hot water and wiped clean between cuts. Store leftovers covered in the refrigerator.