Whisk the egg yolks in a pan with the sugar until smooth. Then whisk in the yogurt.
Heat the pan gently whisking from time to time to ensure no lumps form and the mixture doesn't get so hot that it curdles. The mixture will thin initially, but keep going until it thickens again - about 15 minutes.
Remove from the heat and add the chocolate. Stir until melted and smooth.
Use immediately as a warm custard sauce or pour into individual pots and leave to set in the fridge for a couple of hours.
Serves 4 as a stand alone dessert or 6 if used as a warm custard sauce to accompany a hot pudding.