In a heatproof bowl, combine the chopped chocolate and butter. Place the bowl over a pot of simmering water (double boiler method) and stir until melted and smooth. Remove from heat and let cool slightly.

In a separate bowl, whisk the egg yolks, sugar, and vanilla extract until pale and creamy. Gradually add the melted chocolate mixture to the egg yolk mixture, stirring until well combined.

In another bowl, beat the egg whites with a pinch of salt until stiff peaks form. Gently fold the beaten egg whites into the chocolate mixture until no white streaks remain.

In a separate bowl, whip the heavy cream until soft peaks form. Carefully fold the whipped cream into the chocolate mixture until fully incorporated.

Spoon the mousse into individual cups or ramekins and refrigerate for at least 2 hours to set.

Before serving, garnish with fresh berries or a dollop of whipped cream if desired.