Steps

Melt chocolate gently using the double boiler method and then cool to ~40°C. Stir egg yolk into cooled chocolate

Using the heavy cream and vanilla extract, make whipped cream with soft peaks. Note that there's no added sugar here because we already have the sweetness from the chocolate and the meringue

Whip the egg white, sugar, and salt together to make a french meringue with stiff peaks

Take about 1/3 of the whipped cream and fold it into the chocolate-yolk mixture. This step is important because the chocolate base is dense and heavy so mixing in a small amount of whipped cream first lightens it, making it easier to gently incorporate the rest of your airy ingredients without deflating them. Don’t worry if you lose some volume here; it’s expected. Add Remaining Whipped Cream: Gently fold in the rest of the whipped cream until just combined. Now that the base is lighter, you can fold in the remaining cream more gently, preserving its airiness. Fold in the Meringue: Gently fold the stiff meringue into the chocolate mixture in two additions, being careful not to deflate the mousse. Adding the meringue last, and in portions, ensures the mousse stays as light and fluffy as possible.

Spoon into a serving dish and chill for at least 2 hours

Optionally top with extra whipped cream, chocolate shavings, or berries before serving.