Mix the egg yolks and half the sugar in a bowl, using a fork. Pour the cream into a pan and bring almost to a boil. Gradually beat the cream into the yolk mixture to make a custard.
Strain the custard into a jug, then divide equally among 3 bowls. Stir a different chocolate into each bowl, adding 1 tablespoon liqueur, if using. Stir until melted.
Divide the plain dark chocolate custard between 6 ramekins. When cool, transfer the dishes to the freezer for 10 minutes to set.
Remove the dishes from the freezer. Stir the white chocolate custard and spoon it over the dark layer in the dishes. Return to the freezer for 10 minutes.
Remove the dishes from the freezer. Stir the milk chocolate custard and spoon into the dishes. Chill in the refrigerator for 3–4 hours until set. About 25 minutes before serving, sprinkle with the remaining sugar and caramelize with a blow torch or under a hot grill. Leave at room temperature until ready to eat, then decorate with chocolate curls, if desired.