01 - Combine crushed cookies and melted butter in a bowl until uniformly mixed. Distribute the mixture evenly among 4 serving cups, pressing down gently to form a compact base layer. Chill in the refrigerator while preparing the mousse.
02 - Gently melt the dark chocolate using a double boiler or microwave in short intervals, stirring continuously until smooth. Allow to cool slightly.
03 - In a separate bowl, beat the egg yolks with sugar and vanilla extract until the mixture becomes thick and pale.
04 - Whip the cold heavy cream to soft peaks using an electric mixer or whisk.
05 - In a clean bowl, whisk egg whites with a pinch of salt until stiff peaks form.
06 - Fold the melted chocolate into the egg yolk mixture until fully incorporated.
07 - Gently fold in the whipped cream, followed by the beaten egg whites, ensuring the mousse remains light and airy.
08 - Spoon the mousse evenly over the chilled cookie bases in the cups. Smooth the surface with a spatula.
09 - Refrigerate for a minimum of 2 hours to allow the mousse to firm up.
10 - Prior to serving, optionally decorate with whipped cream, chocolate shavings, or fresh berries.