Place the milk chocolate and espresso in a heatproof bowl. Sit the bowl over a saucepan of gently simmering water, ensuring the base of the bowl does not touch the water. Stir until the chocolate has melted and the mixture is smooth. Remove from the heat and set aside to cool.
In a clean bowl, beat the egg yolks and caster sugar with a cordless hand mixer on high speed for 1–2 minutes, or until the mixture is thick, pale and creamy. In a separate bowl, whip the thickened cream to stiff peaks. Gently fold the whipped cream into the egg yolk mixture, then carefully fold in the cooled chocolate mixture until just combined.
Wash and thoroughly dry the beaters. In a clean, grease-free bowl, whisk the egg whites to firm peaks. Gently fold the egg whites into the chocolate mixture in two or three additions, taking care not to overmix and lose the air.
Divide the mousse evenly among six serving glasses or ramekins. Cover and refrigerate for 4–6 hours, or until set.
Just before serving, sprinkle with the grated dark chocolate.
