01 - Melt dark chocolate using a double boiler or microwave in short bursts, stirring until smooth. Allow to cool slightly. Vigorously whisk the egg yolk with sugar in a mixing bowl until pale and creamy. Incorporate the melted chocolate into the yolk mixture gently. Whip heavy cream with a whisk or electric mixer until soft peaks form, then fold into the chocolate base. Evenly distribute the mixture across the bottom of six small serving cups. Chill in the refrigerator while continuing with the next mousse layer.
02 - Repeat the blending process with milk chocolate, egg yolk, sugar, and cream in a clean bowl. Once completely smooth and aerated, layer gently on top of the set dark chocolate base. Return cups to refrigerator to chill.
03 - Replicate the mousse preparation with white chocolate, the final egg yolk, sugar, and cream. Fold the whipped cream into the white chocolate blend, ensuring a light texture. Carefully spoon or pipe the mixture onto the milk chocolate layer. Refrigerate all cups for a minimum of 1 hour until fully set.
04 - Just before serving, adorn with shaved chocolate or a light dusting of cocoa powder if desired.