01 - Combine crushed cookies with melted butter in a small bowl until evenly mixed. Divide the mixture into four serving cups, pressing gently to form a firm, even base. Chill in the refrigerator while preparing the mousse.
02 - Melt the chopped dark chocolate using a double boiler or microwave in short bursts, stirring until smooth. Allow to cool slightly.
03 - Whisk the egg yolks with sugar and vanilla extract in a medium bowl until the mixture becomes thick and pale.
04 - Whip the cold heavy cream in a separate bowl until soft peaks form.
05 - In a clean bowl, beat the egg whites with a pinch of salt until stiff peaks develop.
06 - Fold the melted chocolate gently into the egg yolk mixture until fully incorporated.
07 - Carefully fold the whipped cream into the chocolate and egg yolk mixture to maintain an airy texture.
08 - Gently fold the beaten egg whites into the mousse mixture until smooth and light.
09 - Spoon the mousse evenly over the chilled cookie bases in the serving cups and smooth the tops.
10 - Refrigerate the assembled cups for a minimum of two hours to allow the mousse to set.
11 - Before serving, optionally adorn with whipped cream, chocolate shavings, or fresh berries as preferred.