Cream butter and sugar in a large bowl. Add egg yolks, one at a time, beating
well after each addition. Beat in the lemon juice, flour, lemon rind and salt. Add
the milk in a steady stream, beating and combining the mixture well.
Beat the egg whites with the cream of tartar in a medium-size bowl until stiff
peaks form. Stir one-fourth of the egg whites into the lemon mixture. Gently, but
thoroughly, fold in the remaining whites.
Pour the mixture into a 1 1/2-quart soufflé dish.
Set the dish in a deep pan and pour boiling water into the pan to come halfway
up the sides of the dish.
Bake for about 50 minutes or until puffed and golden brown on the top.