01 - Combine crushed cookies with melted butter in a bowl, mixing thoroughly. Divide the mixture evenly among four serving cups, pressing gently to create a firm base layer. Refrigerate to set while preparing the mousse.
02 - Gently melt the dark chocolate in a heatproof bowl over simmering water or in brief microwave intervals, stirring until smooth. Allow the chocolate to cool slightly.
03 - In a separate bowl, beat egg yolks with granulated 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 gently into the egg yolk mixture until fully incorporated.
07 - Gently fold the whipped cream into the chocolate and egg yolk mixture, maintaining airiness.
08 - Carefully fold the stiff egg whites into the mousse mixture until smooth and homogeneous.
09 - Divide the mousse over the prepared cookie bases in the cups, smoothing the surface. Refrigerate for a minimum of two hours to set.
10 - Before serving, optionally decorate with whipped cream, chocolate shavings, or fresh berries.