Cream butter and sugar until pale in colour and fluffy in appearance. Add the eggs and beat well. Sift in the flour and fold in with the milk. Fold in the lemon zest and the chocolate. Spoon into a oven proof dish and sprinkle with the extra sugar. Combine water and lemon juice and pour over the top of the pudding. Bake 30 -40 minutes until pudding is brown and firm on top.