Melt the chocolate in a bowl set over a pan of simmering water.
Blend flour, sugar, cocoa, milk, egg, baking powder, and nutmeg in a processor for one minute.
Mix in the melted chocolate at high speed and then fold in the hazelnuts.
Spoon the batter into a greased 1.2 litre pudding basin.
Cover with a lid or pleated greased foil tied with string.
Place the basin on a rack inside a large pan.
Fill the pan with boiling water until it reaches three-quarters up the basin side.
Cook over medium heat for 1 to 1.5 hours until a skewer comes out clean.
Cool for ten minutes, loosen the edges with a knife, and invert onto a dish.
Serve the pudding with vanilla custard.