Melt chocolate and butter together. Sift flour, baking powder and salt together.
Beat egg; add sugar gradually and continue beating till mixture is creamy. Add melted chocolate and mix thoroughly.
Stir dry ingredients alternately with milk mixture, beating until smooth after each addition. Fill the greased pudding basin 2/3 full. Cover tightly with a greaseproof paper lid. Secure lid with string.
Place the basin inside a large pot and pour water in the pot until the pudding is half immersed in water. Bring to a boil and then simmer for 1½ hours.