Reduce the wine. Pour the red wine into a small saucepan and set over medium heat. Simmer gently until it reduces to about 1/3 cup (80 ml), 8–12 minutes. This concentrates the flavor. Set aside to cool slightly until warm, not hot.

Melt the chocolate. Place chopped chocolate in a heatproof bowl. Set the bowl over a pot of barely simmering water (don’t let the bottom touch the water). Stir until smooth, then remove from heat. Let it cool until just warm to the touch.

Whip the cream. In a cold bowl, whip the heavy cream to soft peaks. You want it thick but still silky. Refrigerate while you prepare the next steps.

Make the egg base. In a clean bowl, whisk the egg yolks with sugar, salt, and vanilla until pale and slightly thick, about 1–2 minutes. Slowly whisk in the warm reduced wine. It may look glossy and thin—this is right.

Combine with chocolate. Whisk the warm chocolate into the egg-wine mixture in two additions, stirring until smooth and uniform. If it starts to seize, warm the bowl gently over the steam again and whisk until glossy.

Whip the egg whites. In a separate clean, grease-free bowl, beat the egg whites to medium peaks. They should stand up but curl at the very tip. This adds lightness and structure.

Fold in the whites. Using a rubber spatula, fold one-third of the egg whites into the chocolate mixture to loosen it. Gently fold in the rest just until no big streaks remain. Be patient and don’t stir aggressively.

Fold in whipped cream. Retrieve the whipped cream and fold it in gently, again in two additions. Stop as soon as the mixture looks mostly uniform and airy.

Portion and chill. Spoon into glasses or ramekins. Cover and refrigerate for at least 2 hours, or up to 24 hours. The mousse will set and the flavors will meld.

Garnish and serve. Top with a dollop of whipped cream, berries, shaved chocolate, or a tiny pinch of flaky salt. Serve chilled.