Cream the butter and sugar. Add the egg and vanilla and blend. Sift in the flour and cocoa. Gradually add the milk, blend to a thick batter and pour into the pudding dish.
For the sauce, mix the cocoa and sugar. Sprinkle this mix over the pudding batter. Carefully pour the hot water over the top. Cook in a 350°F/180°C oven for 35 - 45 minutes.