1. Cream butter with all but 2 tbsp sugar.
2. Beat in egg yolks and lemon peel.
3. Stir in flour, just until mixed.
4. Add milk and lemon juice.
5. Stir well.
6. In another bowl, beat egg whites with salt until soft peaks form.
7. Gradually beat in remaining 2 tbsp sugar.
8. Beat until stiff peaks form.
9. Fold egg whites gently into lemon mixture.
10. Spoon batter into buttered 8 inch baking dish.
11. Place baking dish in a larger pan of very hot water (to come halfway up the sides of the baking dish).
12. Bake in preheated 350F (180C) degree oven for 40-45 minutes or until puffed and browned on top.
13. Cover with aluminum foil half way through baking time if cake is browning too quickly.
14. To serve, cut into squares and dust with icing sugar.
15. Serve warm or cold.