Melt the chocolate and butter. Set a heatproof bowl over a saucepan of simmering water — the bowl must not touch the water. Add the chopped chocolate and the butter and stir until smooth and glossy, then take the bowl off the heat and let it cool for 5 minutes. The chocolate should be warm, not hot — hot chocolate deflates the egg whites.

Whisk in the egg yolks. Whisk the egg yolks into the warm chocolate one at a time, then stir in the vanilla extract and the salt. The mixture will thicken slightly and turn glossy — that is the mousse base.

Whip the cream. In a cold bowl, whip the cold heavy cream with a hand mixer or whisk until it holds soft peaks — the cream should flop over when you lift the whisk, not stand up stiff. Over-whipped cream makes a grainy mousse.

Whip the egg whites. In a separate clean, dry bowl, beat the egg whites with a hand mixer until foamy, then add the sugar a spoonful at a time and beat until the whites hold stiff, glossy peaks. Any grease or yolk in the bowl stops the whites from whipping — wipe the bowl clean first.

Fold the whites into the chocolate. Stir one-third of the whipped whites into the chocolate base to lighten it, then add the rest and fold gently with a spatula — cut down through the middle, scoop up the sides, and turn the bowl. Stop as soon as no white streaks remain; over-folding knocks the air out.

Fold in the whipped cream. Add the whipped cream and fold the same gentle way, just until combined. The mixture should look light, moussey and evenly chocolate-colored, with no streaks of cream or white.

Spoon into ramekins and chill. Divide the mousse among 6 ramekins or small glasses, smooth the tops, cover with plastic wrap and chill for at least 2 hours — overnight is even better. The mousse sets as it chills.

Serve. Top each mousse with a spoonful of whipped cream, a few fresh berries and some chocolate shavings. Serve cold, straight from the fridge.