Chocolate Mousse Cake

Prep the pan

1. Heat oven to 350°F. Line the bottom of a 9-inch springform pan (or cake pan) with parchment. Lightly grease the sides.
2. If using a springform: This batter is thin and some pans like to leak. Set the pan on a sheet pan, or wrap the outside bottom with a layer of foil if your pan is a known troublemaker.
3. If using a standard cake pan: Line the bottom, then line the sides with parchment strips, or make a parchment sling with overhang so you can lift the chilled cake out cleanly.

Bake the cake base

1. In a medium bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
2. Add egg, buttermilk, oil, and vanilla. Whisk until smooth.
3. Whisk in the hot coffee (or hot water). The batter will be thin. That is correct and also slightly alarming the first time.
4. Pour into the prepared pan and bake for 16 to 20 minutes, until the center springs back lightly and a toothpick comes out with a few moist crumbs.
5. Cool the cake completely in the pan. If you are using a springform, you can leave it assembled for the mousse step.

Make the mousse

1. Melt the chocolate: Put chopped chocolate in a heatproof bowl. Set aside.
2. Heat the eggs and sugar gently: For the most foolproof method, set the bowl over a saucepan of barely simmering water (double boiler). Whisk eggs and sugar constantly for 3 to 5 minutes, until the mixture is warm, slightly thickened, and foamy, about 160°F if you have a thermometer. It should feel warm, not hot. If you prefer using a saucepan directly, keep the heat very low and whisk constantly so the eggs do not scramble.
3. Combine: Immediately pour the warm egg mixture over the chopped chocolate. Let sit 1 minute, then whisk until smooth and glossy. Whisk in vanilla and salt.
4. Cool: Let the chocolate mixture cool to lukewarm, about 85 to 90°F, or just slightly warm to the touch. If it is too warm, it can melt the whipped cream and you will lose that airy mousse lift.
5. Whip the cream: In a clean bowl, whip cold heavy cream to medium peaks. You want it billowy but still flexible.
6. Fold: Fold 1/3 of the whipped cream into the chocolate mixture to lighten it, then gently fold in the remaining whipped cream until no streaks remain.

Assemble and chill

1. Spoon the mousse over the cooled cake base and smooth the top with an offset spatula or the back of a spoon.
2. Cover and refrigerate until fully set, at least 6 hours (overnight is even better). You can make it 1 day ahead with zero regret.

Serve

1. If using a springform pan, run a thin knife around the edge, unclasp, and lift off the ring.
2. If using a standard cake pan, use the parchment overhang to lift the cake out, then peel the parchment away and transfer to a serving plate.
3. Dust with cocoa powder, add chocolate shavings, and a tiny pinch of flaky salt if you like that sweet-salty moment. Add berries or a spoon of whipped cream if you want a little contrast.
4. Slice with a hot, clean knife for sharp layers.