In a glass bowl, combine the chocolate and 1/4 cup heavy cream. Melt in the microwave in 20-second intervals, stirring between each interval, or set over a pot of simmering water. Stir until smooth.

Whisk in the egg yolks, one at a time, until fully incorporated. Stir in the vanilla and salt.

Using an electric mixer, beat the egg whites until stiff peaks form. Add a spoonful of the whipped egg whites to the chocolate mixture to lighten it, then gently fold in the remaining egg whites until no streaks remain.

Cover and chill the mousse until fully set, about 2 hours.

Once chilled, add 1/2 cup heavy cream to the mousse and whip until soft and fluffy. Gradually add more cream, whipping until incorporated, and your desired consistency is reached.

Spoon into serving glasses. Serve immediately, or refrigerate until ready to serve. Finish with chocolate shavings and flaky sea salt.