01 - Melt dark chocolate in a heatproof bowl set over simmering water or in the microwave in short bursts, stirring until smooth. Allow to cool slightly. In a mixing bowl, whisk egg yolk with sugar until pale and creamy. Gradually fold the melted chocolate into the egg mixture. Whip cold heavy cream to soft peaks, then gently fold into the chocolate mixture. Divide evenly among six small serving cups, smoothing the surface. Refrigerate while starting the next layer.
02 - Melt milk chocolate in a heatproof bowl over simmering water or using the microwave. Cool slightly. In a clean bowl, whisk egg yolk and sugar until pale and light. Fold in melted milk chocolate, followed by whipped cream beaten to soft peaks. Spoon gently over the set dark chocolate layer, distributing evenly. Return cups to the refrigerator to chill.
03 - Melt white chocolate over low heat or in the microwave, stirring frequently. In a separate bowl, whisk egg yolk with sugar until creamy. Blend in melted white chocolate, then fold in cream whipped to soft peaks. Top the milk chocolate layer with the white chocolate mousse, smoothing or piping for a clean finish. Refrigerate all assembled cups for at least 1 hour or until fully set.
04 - Before serving, garnish each cup with shaved chocolate or dust lightly with cocoa powder, if desired.