Prep your pan and oven. Heat oven to 350°F (175°C). Line the bottom of a 9-inch springform pan with parchment and lightly grease the sides. This will make unmolding easy and keep edges neat.

Mix dry ingredients. In a medium bowl, whisk flour, cocoa powder, sugar, baking powder, and salt until evenly combined. This prevents lumps and gives you a tender crumb.

Combine wet ingredients. In another bowl, whisk egg, milk, oil (or melted butter), and vanilla until smooth. Pour into the dry mix and whisk just until combined. Stir in the hot coffee or water to thin the batter; it will look loose—this is right.

Bake the base. Pour the batter into the prepared pan and bake for 16–20 minutes, until the center springs back lightly and a tester comes out with a few moist crumbs. Cool in the pan on a rack. Once warm, run a knife around the edge to loosen, but keep the cake in the pan for assembly.

Make a simple ganache starter. Place the chopped chocolate and butter in a heatproof bowl. Warm 1/2 cup of the heavy cream until steaming (not boiling), then pour over the chocolate. Let sit 2 minutes, then stir until smooth and glossy. Stir in vanilla and a pinch of salt. Cool to just above room temperature—still fluid but not warm.

Optional: Whip egg whites for lift. If using, beat egg whites with cream of tartar to soft peaks. Add 1 tablespoon powdered sugar and beat to medium peaks. This gives extra air to the mousse, but you can skip this step if you prefer egg-free.

Whip the cream. In a cold bowl, whip the remaining 1 1/4 cups heavy cream with 1–2 tablespoons powdered sugar to soft-medium peaks. Don’t overwhip. You want silky ribbons that hold shape but remain supple.

Fold the mousse. Stir a spoonful of whipped cream into the cooled chocolate to loosen it. Gently fold in the rest of the cream with a spatula, keeping as much air as possible. If using egg whites, fold them in last, in two additions, until the mixture is uniform and airy.

Assemble. Ensure the cake base is fully cool. If it domed, level the top with a serrated knife so the mousse sits flat. Keep the cake in the springform pan and latch the ring snugly. Spread the mousse over the cake and smooth the top. Tap the pan lightly on the counter to pop large air bubbles.

Chill to set. Cover the pan and refrigerate at least 4 hours, preferably overnight. The mousse firms up and slices cleanly when fully set.

Finish and serve. Run a thin knife around the inside of the pan, then release the springform ring. Dust the top with cocoa powder or add chocolate shavings. Slice with a hot, dry knife for clean edges. Serve chilled or slightly cool.