Prep the equipment. Set out a heatproof bowl, a saucepan, two mixing bowls (one very clean and dry for egg whites), and a whisk or electric mixer. Make sure no grease touches the egg whites.

Melt the chocolate. Chop the dark chocolate into small pieces. Place it in a heatproof bowl with the butter (if using). Set the bowl over a pot with a little simmering water, making sure the bowl doesn’t touch the water. Stir gently until smooth, then remove from heat. Let cool to lukewarm.

Separate the eggs. Crack the eggs while the chocolate cools. Place the yolks in a small bowl and the whites in a large, clean mixing bowl. Add a pinch of salt to the whites.

Flavor the base. Whisk the egg yolks into the cooled chocolate one at a time. Add vanilla and espresso powder if using. The mixture should be glossy and thick.

Whip the egg whites. Using a clean whisk or mixer, beat the egg whites on medium speed until foamy. Gradually rain in the sugar while beating. Increase to medium-high and whip to soft peaks that hold their shape but curl at the tip.

Lighten the chocolate. Stir a generous spoonful of whipped whites into the chocolate mixture to loosen it. This makes folding easier and prevents deflating the rest.

Fold gently. Add the remaining whites in 2–3 additions. Use a wide spatula to fold with slow, sweeping motions, turning the bowl as you go. Stop when no obvious streaks remain. It’s okay if a few small white wisps are visible—better slightly streaky than overmixed.

Portion and chill. Spoon the mousse into small ramekins, cups, or glasses. Smooth the tops if you like. Cover and chill for at least 2 hours, or until softly set. For a firmer set, chill 4–6 hours.

Garnish and serve. Top with shaved chocolate, a dusting of cocoa, or fresh berries. Serve chilled. The texture should be airy and melt-in-your-mouth.