Melt the Chocolate: In a heatproof bowl over simmering water, melt the chopped chocolate and butter together. Stir occasionally until smooth and glossy. Remove from heat and let it cool slightly.

Whisk the Egg Yolks: In a separate bowl, whisk the egg yolks and one tablespoon of sugar until thick and pale. Stir in the melted chocolate and vanilla extract. Set aside.

Beat the Egg Whites: In another clean bowl, add the remaining egg whites and a pinch of salt. Beat with an electric mixer until soft peaks form. Gradually add the other tablespoon of sugar and beat until glossy, firm peaks form.

Fold It All Together: Gently fold the egg whites into the chocolate mixture in two additions. Be careful not to deflate the mixture too much.

Whip the Cream: In a separate bowl, whip the heavy cream until soft peaks form, then fold it into the chocolate mixture.

Assemble the Cake: Pour the mousse mixture into serving cups or a cake mold. Chill in the refrigerator for at least four hours or overnight until fully set.