Melt the chocolate in a bowl set over a pan of simmering water.
Combine the remaining ingredients, except for the hazelnuts, in a food processor or mixer and blend for one minute at low speed.
Add the melted chocolate and blend for one minute at high speed. Fold in the hazelnuts.
Lightly grease a 1.2 litre/2 pint pudding basin with margarine. Spoon the batter into the basin. Cover with a lid or a pleated greased sheet of foil tied securely with string.
Spoon the batter into the basin. Cover with a lid or a pleated greased sheet of foil tied securely with string.
Place the basin on a rack set in the bottom of a pan.
Pour boiling water into the pan until it comes three-quarters the way up the side of the basin.
Cook over a medium heat for about 1-1½ hours or until a knife or skewer inserted into the centre of the pudding comes out clean.
Remove the pudding from the pan and allow to cool for ten minutes. Run a knife around the edge to loosen and invert on to a serving dish.
Serve with vanilla custard.