01 - Combine crushed cookies and melted butter in a small bowl until evenly mixed. Distribute the mixture into 4 serving cups, pressing gently to form a uniform layer. Chill in refrigerator while preparing mousse.
02 - Gently melt dark chocolate in a heatproof bowl set over simmering water or use microwave in short intervals, stirring until smooth. Allow to cool slightly.
03 - In a medium bowl, beat egg yolks with sugar and vanilla extract until pale and thickened.
04 - Whip cold heavy cream to soft peaks in a separate bowl.
05 - Whip egg whites with a pinch of salt until stiff peaks form in a clean bowl.
06 - Stir melted chocolate into the egg yolk mixture until fully incorporated.
07 - Carefully fold whipped cream into the chocolate mixture until smooth.
08 - Gently fold in beaten egg whites until mousse is airy and homogenous.
09 - Spoon mousse over cookie bases in each cup, smoothing tops evenly.
10 - Refrigerate for a minimum of 2 hours or until mousse is firm.
11 - Before serving, decorate with whipped cream, chocolate shavings, or fresh berries if desired.