01 - Gently melt the dark chocolate over a bain-marie or in 20-second increments in the microwave, stirring until smooth. Allow to cool slightly. In a separate bowl, vigorously whisk the egg yolk and sugar until pale and thick. Fold the cooled, melted chocolate into the yolk mixture until combined. In another bowl, whip the heavy cream to soft peaks, then gently incorporate into the chocolate mixture using a spatula. Divide the mousse evenly between 6 serving cups as the base layer. Refrigerate the cups while preparing the next mousse.
02 - Repeat the melting, whisking, and folding process using the milk chocolate, egg yolk, sugar, and cream. Once the mixture is airy and smooth, carefully spoon or pipe over the set dark chocolate layer in each cup. Return to the refrigerator to chill before adding the final layer.
03 - Follow the same technique with the white chocolate, remaining egg yolk, sugar, and cream. Gently layer or pipe the white chocolate mousse on top of the milk chocolate layer. Refrigerate the assembled cups for at least 1 hour, or until fully set.
04 - Before serving, garnish each cup with shaved chocolate or lightly dust with cocoa powder if desired.