Cream butter. Add sugar, flour, lemon juice, and lemon rind. Beat egg yolks, and mix with milk, then stir them into the sugar, flour and lemon mixture, and add salt. Beat egg whites until stiff, and then gently fold into other mixture.

Pour into 6 small-size custard cups. Bake in a water bath, in a 350 degree oven, for 45 minutes or until done. Garnish with a fresh mint leaf. When done, each custard cup will have a lemon custard in the bottom with sponge cake on top.