Break each chocolate into a separate heatproof mixing bowl and melt in the microwave, or place one at a time over a pan of gently simmering water, then remove the bowl from the pan and set aside to cool slightly.
Put the eggs and sugar in a medium heatproof mixing bowl. Set the bowl over a pan containing 2.5cm (1in) of gently simmering water and whisk with an electric hand whisk on low speed for 4-5 minutes. The mixture will become thickened and paler in colour. Remove the bowl from the pan and heat, then continue whisking for a minute or 2 to cool the mixture. Set aside.
Whisk the cream to very soft peaks. Fold one third of the cream into the cooled dark chocolate, followed by two thirds of the egg yolk mixture, then the liqueur, brandy or rum. Fold the rest of the cream into the white chocolate, then the rest of the egg yolk mixture.
Layer the two mousses into four glasses and decorate with grated chocolate or cocoa. Chill for 1 hour before serving.
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