Preheat your oven to 350°F (175°C). In a bowl, combine the chocolate cookie crumbs with the melted butter until they resemble damp sand.

Press this mix into the bottom of a springform pan to form an even crust. Bake for about 10 minutes and let it cool.

In a heatproof bowl over a simmering pot, melt the semi-sweet chocolate, stirring until smooth. Allow it to cool slightly.

Meanwhile, in another bowl, whip the heavy cream to soft peaks.

In a separate bowl, whisk together the egg yolks and sugar until pale and thick. Stir in the vanilla extract and the melted chocolate.

Fold the whipped cream into the chocolate mixture until combined.

In a clean bowl, whip the egg whites with a pinch of salt until soft peaks form.

Gently fold the egg whites into the chocolate mousse mixture.

Pour this mixture over your cooled crust and refrigerate for at least 4 hours, or overnight.

When ready to serve, whip the remaining heavy cream until soft peaks form.

Spread this on top of your chilled mousse cake, adding chocolate shavings or cocoa powder for garnish.