Pennsylvania Amish Lemon Sponge Pie

1 cup sugar
2 tbls. butter
3 eggs yolks, beaten
2 tbls. flour
1 lemon - juice and grated rind
1 1/2 cup hot milk
3 egg whites, stiffly beaten
1 tsp. vanilla
1 unbaked pie shell
Cream the sugar and butter. Add the egg yolks. Add the flour, lemon and hot milk, mixing well. Fold in the egg whites. Pour into the pie shell and bake at 325º for 45 minutes.