Let’s build this incredible Oreo Cake layer by layer. First, prepare for the cake. Preheat your oven to 350°F (176°C). Grease your three 6-inch round cake pans thoroughly. Dust them with cocoa powder (tapping out excess) instead of flour to avoid white marks on the dark cake. Line the bottom of each pan with a circle of parchment paper.

In the large bowl of your stand mixer fitted with the paddle attachment, combine all the dry cake ingredients: all-purpose flour, sifted Oreo crumbs, granulated sugar, sifted Dutch-processed cocoa, sifted black cocoa powder, baking soda, baking powder, and salt. Stir them on low speed just to combine everything evenly. Sifting the cocoas and Oreo crumbs beforehand prevents lumps.

In a separate medium bowl, whisk together the wet ingredients: buttermilk, vegetable oil, eggs, and vanilla extract. If your hot water or coffee is very hot, whisk the other wet ingredients first, then slowly stream in the hot liquid while whisking continuously to temper the eggs and prevent scrambling.

Pour the combined wet ingredients into the bowl containing the dry ingredients. Mix on medium speed for about 2-3 minutes, scraping down the sides of the bowl occasionally. The batter will be very thin and liquidy; this is correct for this type of moist chocolate cake.

Carefully divide the thin batter evenly among the three prepared 6-inch pans. Using a kitchen scale is the most accurate way to ensure even layers. Bake in the preheated oven for approximately 30-35 minutes. Start checking around the 30-minute mark. The cakes are done when a wooden skewer or cake tester inserted into the center comes out mostly clean, perhaps with a few moist crumbs attached.

Allow the cakes to cool in their pans on wire racks for about 10 minutes. Then, carefully run a knife around the edges and invert the cakes onto the wire racks. Peel off the parchment paper and let them cool completely. Once fully cooled, the delicate task begins: use a long, sharp serrated knife to carefully slice each cake layer horizontally in half, creating a total of six thin cake layers. Work slowly and keep the knife parallel to the work surface.

Now, for the Oreo Swiss Meringue Buttercream (SMBC). Place the egg whites and granulated sugar into the clean, grease-free bowl of your stand mixer. Whisk them together by hand just until combined. Create a double boiler by setting the mixer bowl over a saucepan filled with a couple of inches of simmering (not boiling) water. Ensure the bottom of the bowl doesn’t touch the water.

Continuously whisk the egg white mixture over the simmering water until the sugar has completely dissolved and the mixture feels smooth (not grainy) when rubbed between your fingertips. This typically takes about 3 minutes. For food safety peace of mind, you can check with an instant-read thermometer – it should reach about 160°F (71°C).

Carefully transfer the hot bowl back to your stand mixer fitted with the whisk attachment. Beat the mixture on medium-high speed until it forms a stiff, glossy meringue and, crucially, the outside of the bowl feels completely cool to the touch. This cooling process can take 5-10 minutes or even longer; be patient, as adding butter to warm meringue will melt it.

Once the meringue is stiff and completely cool, switch to the paddle attachment. With the mixer running on low or medium-low speed, begin adding the room-temperature (cool but pliable) cubed butter, one or two cubes at a time. Allow each addition to incorporate slightly before adding the next.

The mixture will likely go through stages of looking curdled, soupy, or separated – this is normal for SMBC! Keep the mixer running on medium speed, and it should eventually come together into a smooth, cohesive buttercream. This can take several minutes of continuous mixing. If it seems stubbornly soupy, chill the bowl for 15 minutes and try beating again. If it seems broken because the butter was too cold, gently warm the outside of the bowl with a hairdryer briefly while mixing.

Once the buttercream is smooth, add the clear vanilla extract (if using) and mix until combined. Remove about 1 cup of this plain white buttercream and set it aside in a separate bowl for piping later. Add the sifted Oreo cookie crumbs to the remaining buttercream in the main bowl. Mix on low speed just until the crumbs are evenly distributed. Don’t overmix, which can break down the crumbs too much or incorporate excess air.

Next, prepare the dark chocolate ganache. Place the finely chopped dark chocolate in a medium microwave-safe bowl. Add the heavy cream. If using the black food color gel for a deeper color, stir it into the cream first. Microwave the mixture in short bursts of 10-15 seconds, stirring well after each interval. Continue until the chocolate is completely melted and the ganache is smooth and shiny. Be careful not to overheat, which can cause the chocolate to seize. Set the ganache aside to cool completely and thicken to a drip consistency (it should coat the back of a spoon but still be pourable) before using.

Finally, assemble the cake! Place one thin cake layer on your cake stand or serving plate. Top with about 2/3 cup of the Oreo buttercream (or slightly less, given the thin layers), spreading it evenly. Sprinkle some of the chopped Oreos over the frosting. Repeat this process with the next four layers: cake, frosting, chopped Oreos. Place the sixth and final cake layer on top.

Apply a thin crumb coat of the Oreo buttercream all over the cake, smoothing it with your offset spatula. Chill the cake in the refrigerator for about 20 minutes to firm up the crumb coat. Frost the outside of the chilled cake with the remaining Oreo buttercream, using your offset spatula and bench scraper to achieve a smooth finish. Chill the cake again for at least 30 minutes until the frosting is firm to the touch.

Carefully apply the cooled and thickened dark chocolate ganache drip. Use a small spoon or squeeze bottle to apply dollops around the top edge, letting some drip down the sides. Pour the remaining ganache onto the top center and spread it evenly with a small offset spatula. Allow the ganache to set for a few minutes (chilling briefly can speed this up).

Fill a piping bag fitted with a 1M (or similar large star) tip with the plain white buttercream you set aside earlier. Pipe decorative rosettes or dollops around the top edge of the cake, on top of the set ganache. If desired, place a mini Oreo cookie in the center of each piped dollop. Chill the cake until ready to serve.