01 - Combine crushed cookies with melted butter until uniformly mixed. Divide the mixture evenly into 4 serving cups, pressing gently to form a compact layer. Chill in refrigerator while preparing mousse.
02 - Gently melt dark chocolate in a heatproof bowl set over simmering water or microwave in short intervals, stirring until smooth. Allow to cool slightly.
03 - Whisk egg yolks with sugar and vanilla extract until thickened and pale in color.
04 - Beat cold heavy cream to soft peaks using an electric mixer or whisk.
05 - Whip egg whites with a pinch of salt to stiff peaks in a clean bowl.
06 - Fold melted chocolate gently into egg yolk mixture until fully incorporated.
07 - Carefully fold whipped cream into chocolate mixture to maintain light texture.
08 - Gently fold the beaten egg whites into the mousse mixture until smooth and airy.
09 - Spoon mousse over cookie bases in cups, smoothing the surface evenly.
10 - Refrigerate for a minimum of 2 hours until fully set.
11 - Before serving, optionally top with whipped cream, chocolate shavings, or fresh berries.