01 - Combine crushed cookies with melted butter until evenly incorporated. Divide the mixture evenly into 4 serving cups, pressing gently to form a firm base layer. Chill while preparing the mousse.
02 - Melt the 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, sugar, and vanilla extract together until mixture is thickened and pale.
04 - Whip cold heavy cream to soft peak consistency.
05 - Whisk egg whites with a pinch of salt until stiff peaks form.
06 - Fold melted chocolate into the egg yolk mixture until homogenous.
07 - Gently fold whipped cream into chocolate mixture, followed by carefully folding in the beaten egg whites to maintain airiness.
08 - Spoon the mousse over the cookie bases in each cup, smoothing the surface. Refrigerate for a minimum of 2 hours to set.
09 - Optionally adorn with whipped cream, chocolate shavings, or fresh berries before serving.