1. Melt the chopped semi-sweet chocolate and unsalted butter together in a heatproof bowl over a double boiler or in short microwave bursts, stirring until smooth. Let cool slightly.

2. In a separate bowl, whisk the egg yolks with the granulated sugar and vanilla extract until pale and slightly thickened.

3. Slowly stir the melted chocolate mixture into the egg yolk mixture until smooth and fully combined.

4. In another bowl, whip the heavy cream until soft peaks form.

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

6. Gently fold the whipped cream into the chocolate mixture until mostly combined.

7. Carefully fold in the beaten egg whites in batches, mixing gently to keep the mousse light and airy.

8. Spoon the mousse into individual serving cups.

9. Refrigerate for 2 to 3 hours, or until set.

10. Top with whipped cream, chocolate shavings, and fresh berries before serving.