I always start by preheating my oven to 350°F and spraying two 8-inch cake pans with nonstick spray. Then I line the bottoms and sides with parchment paper for a clean release. This small step saves so much frustration later when it's time to unmold the layers.
In a large bowl, I sift together the flour, black cocoa powder, sugar, baking powder, baking soda, and salt. Sifting isn't optional here since black cocoa tends to clump. This step ensures the batter bakes up smooth instead of streaky.
In a separate bowl, I whisk the buttermilk, oil, eggs, and vanilla together until fully combined. Having everything at room temperature here really does matter, since cold ingredients can make the batter separate or curdle slightly.
I make a well in the dry ingredients and pour in the wet mixture, stirring until just combined. Then I mix in the hot coffee last. The batter will look thin, almost like chocolate milk, but that's exactly what gives this cake its ultra-moist texture.
I divide the batter evenly between my prepared pans and bake for 30 to 33 minutes, checking with a toothpick around the 30-minute mark. Once it comes out clean, I pull the cakes out and let them cool completely in the pans before removing them.
While the cakes cool, I add blackberries, sugar, lemon juice, lemon zest, and a cinnamon stick to a saucepan and bring it to a boil for five to six minutes. Then I stir cornstarch into the water, add it to the berries, and cook until thickened. I remove the cinnamon stick and chill the compote for an hour.
In a mixing bowl, I beat softened cream cheese and butter together, then sift in the powdered sugar, black cocoa powder, and salt. I beat everything until smooth, scrape down the bowl, then mix in vanilla for a final pass. If you're craving another chocolate-forward dessert while this chills, my chocolate chip cake recipe uses a similar rich cocoa base and comes together just as easily.
Once the cakes are fully cool, I level the domed tops with a serrated knife. I spread a small amount of frosting on the cake plate, place my first layer down, then pipe a ring of frosting near the edge to create a dam before filling the center with blackberry compote.
I place the second cooled layer on top upside down for a flatter surface, then apply a thin crumb coat of frosting over the whole cake. After chilling it for 20 minutes in the fridge, I finish frosting with the remaining black cocoa buttercream.
Finally, I top the cake with chocolate skulls, a scattering of fresh blackberries, and a few crushed dried rose petals. This is the fun part where the cake really transforms into a showstopping Halloween centerpiece.