Preheat the oven to 180°C (350°F). In a mixing bowl, combine the crumbled digestive biscuits and melted butter. Press the mixture into the bottom of a springform pan to form a base.

Melt the dark chocolate using a double boiler. Allow it to cool slightly.

In another bowl, whisk the egg yolks with the sugar until pale and creamy. Stir in the melted chocolate and vanilla extract.

In a separate bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the chocolate mixture.

Fold in the whipped cream until fully combined.

Pour the chocolate mousse over the biscuit base in the springform pan. Smooth the top with a spatula.

Refrigerate for at least 4 hours or until set.

Once set, carefully remove the springform pan. Slice and serve chilled.