Put rhubarb in saucepan, add water and sugar.
Cover and cook until tender.
(Makes about 1 cup thick sauce.) Sift flour, measure and resift with remaining dry ingredients.
Add to the creamed butter and mix well.
Add the beaten egg yolks and lemon juice and beat until light and fluffy.
Add cooked rhubarb, and mix well.
Beat the egg whites until stiff.
Fold lightly but thoroughly into the rhubarb mixture.
Pour into custard cups and set in a pan of hot water to come almost to the top of the cups.
Bake in a moderately slow oven (325° F.) for 30 minutes.
Sauce will separate and remain at bottom with a fluffy cake-like topping.