Preheat oven to 350F (180C).
Butter 8-inch (2 L) square glass baking dish, or ramekins.
In bowl, sift together sugar and flour; set aside.
Place egg whites in bowl of stand mixer fitted with whisk attachment. Beat on high speed for 4 to 5 minutes or until stiff peaks form. Transfer whites to another bowl.
Add egg yolks to same bowl and return to stand mixer. Using whisk attachment beat on medium-high speed until yolks are thick and lightened in colour. Reduce speed to medium and add butter, lemon zest and juice; beat for 1 more minute. With machine running on medium speed, alternately beat in flour/sugar mixture and milk, making 5 additions of flour and 4 of milk.
Remove bowl from mixer. Fold in egg whites. Transfer to prepared baking dish. Place dish in large shallow roasting pan. Add hot water to half way up side of baking dish.
Bake in centre of oven for 50 to 55 minutes. Cool for 5 minutes. Top two-thirds will be cake consistency and bottom third will be pudding consistency.