01 - Melt the dark chocolate using a bain-marie or in the microwave with short intervals. Allow to cool slightly. Whisk the egg yolk with sugar until pale and creamy, then fold in the melted chocolate. Whip the heavy cream to soft peaks and gently incorporate into the chocolate mixture. Distribute evenly among six serving cups and refrigerate while preparing the next mousse.
02 - Repeat the melting, whisking, and folding process with the milk chocolate, egg yolk, sugar, and cream. Once the mixture is smooth and aerated, spoon gently over the chilled dark chocolate base. Return the cups to the refrigerator to set this layer.
03 - Melt the white chocolate, then follow the same method by combining with the egg yolk, sugar, and softly whipped cream. Pipe or spoon the white chocolate mousse atop the milk chocolate layer. Chill all assembled cups for at least 1 hour or until each layer is fully set.
04 - Prior to serving, top each mousse cup with shaved chocolate or a dusting of cocoa powder, according to preference.