Preheat the oven to 350°F (175°C).

In a mixing bowl, combine crushed chocolate cookies and melted butter.

Press the mixture into the bottom of a springform pan to form the cake base.

Bake for 10 minutes, then remove and let cool.

In a double boiler, melt the chopped dark chocolate until smooth. Remove from heat and let cool slightly.

In a separate bowl, whip 1 cup of heavy cream until soft peaks form.

In another bowl, beat the egg yolks with sugar and vanilla until pale and creamy.

Fold the melted chocolate into the egg yolk mixture, then gently fold in the whipped cream.

In a clean bowl, whip the egg whites until stiff peaks form, then fold into the chocolate mixture.

Pour the mousse filling over the cooled cake base in the springform pan.

Smooth the top with a spatula and refrigerate for at least 4 hours or until set.

Once set, carefully remove the sides of the springform pan.

Slice and serve chilled, optionally garnished with whipped cream or chocolate shavings.