Preheat oven to 325 degrees F. Lightly butter a 9"x2" round pan or six 6-ounce custard cups or ramekins.

In a bowl using the back of a wooden spoon combine and mash together butter, sugar and salt until crumbly. Beat in egg yolks. Add flour and mix until smooth. Gradually beat in lemon juice and zest. Stir in milk.

In another bowl, beat egg whites on medium-high speed until stiff but still moist (they will look a little curdly, but don't worry). Gently whisk the whites into the milk mixture, blending just until no large lumps of whites remain.

Ladle (do not pour) the batter into the prepared pan or cups; it is okay for it to reach the top. Bake in a water bath until a knife inserted in the center comes out nearly clean, 30 to 40 minutes for both small and large custards. Let stand for 10 minutes in the water bath. During the baking, the batter will divide into a layer of quivery lemon custard on the bottom and a light, spongy cake on top.

Serve warm, at room temperature or chilled, in the mold or turned out. Delicious served with fruit sauces such as raspberry.

For Orange Sponge Custard, prepare Lemon Sponge Custard, above, reducing the lemon juice to 2 tablespoons, adding 1/4 cup strained fresh orange juice, and substituting the grated zest of 1 orange for the lemon zest.