Chill your tools. Place your mixing bowl and beaters in the fridge for 10 minutes. Cold tools help cream whip faster and hold better.

Melt the chocolate. In a heatproof bowl, add the chopped chocolate, butter, and espresso powder if using. Set the bowl over a pot of barely simmering water (don’t let it touch the water) and stir until smooth. Remove from heat and let it cool to lukewarm. It should be glossy and slightly warm, not hot.

Whip the cream. In the chilled bowl, add the heavy cream and 1 tablespoon of sugar. Beat until you get soft peaks—creamy, not stiff. Set aside in the fridge.

Make the egg base. In a clean bowl, whisk the egg yolks with 1 tablespoon of sugar, vanilla, and a pinch of salt until thick and pale, about 1 minute by hand. Stir this into the cooled chocolate until fully blended. If the chocolate looks too thick, add 1 tablespoon of cream to loosen it.

Whip the egg whites. In another clean, dry bowl, beat the egg whites until foamy. Slowly sprinkle in the remaining 1 tablespoon of sugar and continue beating until you have glossy soft peaks. Don’t overdo it; you want them supple, not dry.

Lighten the chocolate. Fold one big spoonful of whipped cream into the chocolate mixture to loosen it. Use a gentle, sweeping motion with a spatula. This step makes the next folds easier.

Fold in the egg whites. Add the whipped egg whites in two additions, folding gently to keep the air. Stop as soon as you don’t see big streaks. A few small streaks are okay; overmixing will deflate it.

Add the remaining cream. Fold in the rest of the whipped cream just until combined. The mousse should look fluffy, smooth, and a shade lighter than before.

Portion and chill. Spoon the mousse into small glasses, ramekins, or cups. Cover and chill for at least 2 hours, or up to 24 hours. It will firm up and become silky.

Garnish and serve. Top with a little whipped cream, shaved chocolate, or berries. A tiny sprinkle of flaky sea salt really wakes up the chocolate.