Melt the dark chocolate in a heatproof bowl over simmering water until smooth.

In a separate bowl, whip the heavy cream with half of the sugar and vanilla extract until soft peaks form. Set aside.

Beat egg whites in a clean bowl until frothy, then gradually add the remaining sugar and continue beating until stiff peaks form.

Gently fold one-third of the whipped cream into the melted chocolate, then carefully fold in the remaining whipped cream and egg whites.

Spoon the mixture into serving cups and chill for at least two hours or overnight.

Serve garnished with extra whipped cream or fresh berries.