Prep your pans and oven: Heat the oven to 350°F (175°C). Grease two 8-inch round cake pans, line the bottoms with parchment, and lightly flour the sides. Tap out any excess flour.

Whisk the dry ingredients: In a large bowl, whisk flour, cocoa powder, granulated sugar, brown sugar, baking powder, baking soda, and salt until no cocoa streaks remain. This helps prevent lumps later.

Combine the wet ingredients: In a separate bowl or large measuring cup, whisk eggs, buttermilk, oil, and vanilla until smooth and slightly frothy.

Make the batter: Pour the wet ingredients into the dry and whisk until almost combined. Slowly add the hot coffee (or hot water) while whisking. The batter will be thin—this is what makes the cake moist.

Divide and bake: Pour the batter evenly into the prepared pans. Bake for 28–32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Cool completely: Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack. Peel off the parchment and cool fully before frosting.

Make the buttercream: Beat the butter on medium speed until creamy and pale, about 2 minutes. Sift together powdered sugar and cocoa. Add half to the butter, beat on low to combine, then add the rest.

Adjust texture and flavor: Add vanilla, salt, and 3 tablespoons of cream. Beat on medium-high for 1–2 minutes until fluffy. If too thick, add a splash more cream. If too soft, add a tablespoon of powdered sugar.

Assemble the cake: Level cake tops if needed. Place the first layer on a stand, spread a generous layer of frosting, and top with the second layer. Apply a thin crumb coat around the sides, chill for 15 minutes, then finish with a thicker, smooth coat.

Serve: Slice with a sharp, warm knife for clean cuts. Add sprinkles, shaved chocolate, or fresh berries if you like.