Melt the Chocolate:

In a heatproof bowl set over a saucepan of simmering water, melt the chocolate and butter together. Stir until smooth, then remove from heat and let cool slightly.

Beat the Egg Yolks:

In a small bowl, whisk the egg yolks and granulated sugar until thick and pale. Gradually add the melted chocolate mixture, stirring until smooth.

Whip the Egg Whites:

In a clean mixing bowl, beat the egg whites until stiff peaks form.

Whip the Cream:

In another bowl, whip the heavy cream until soft peaks form. Add the vanilla extract and gently mix.

Fold the Mixtures Together:

Fold the whipped cream into the chocolate mixture until combined. Then gently fold in the egg whites, being careful not to deflate the mousse.

Chill:

Spoon the mousse into serving glasses or bowls. Refrigerate for at least 2 hours, or until set.

Serve and Garnish:

Serve chilled, topped with whipped cream or chocolate shavings if desired.