Lemon Sponge Pie

2 tablespoons butter
1 cup granulated sugar
3 eggs, separated
4 tablespoons flour
1/2 teaspoon salt
Juice and rind (grated) of 1 lemon
1 1/2 cups hot milk

Cream butter; add sugar and egg yolks. Beat until light. Stir in flour and salt. Add lemon and hot milk. Fold in beaten egg whites. Bake in 9-inch unbaked crust at 400 degrees F for 10 minutes; set at 350 degrees F for 15 or 20 minutes more.