Place the chopped chocolate and butter in a heatproof bowl set over a saucepan of simmering water (bain-marie). Stir occasionally until melted and smooth. Remove from heat and cool to room temperature.

In a medium bowl, whisk the egg yolks with 1 tablespoon of granulated sugar until the mixture is pale yellow and slightly thickened.

Gradually incorporate the cooled chocolate mixture into the egg yolks, whisking constantly until combined.

In a separate chilled bowl, whip the heavy cream and vanilla extract until soft peaks form. Gently fold the whipped cream into the chocolate mixture using a rubber spatula.

In a clean, dry bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining 1 tablespoon of sugar and continue beating until stiff, glossy peaks form.

Fold one-third of the whipped egg whites into the chocolate base to lighten the texture. Carefully fold in the remaining egg whites in two batches until no white streaks remain, taking care not to deflate the mixture.

Divide the mousse into individual ramekins or glasses. Cover and refrigerate for at least 4 hours to allow the structure to set.