Preheat the oven to 325°.
In a large bowl, cream the butter, sugar, salt, and lemon zest.
Add the egg yolks, one at a time, scraping down the sides of the bowl to incorporate thoroughly.
Add the flour and mix until smooth.
Gradually stir in the lemon juice, then the milk.
2. In a clean, dry bowl, whip the egg whites until they form stiff peaks.
Do not overbeat.
3. Gently fold the egg whites into batter, just until no large lumps of white remain.
4. Ladle the batter into 6 lightly buttered 6-ounce ramekins.
Set the ramekins in a larger pan.
Place the pan in the oven and pour enough hot water into the pan to reach halfway up the sides of the ramekins.
Bake for 30 to 40 minutes, or until a knife inserted into the center of the cake comes out clean.
5. Let the cakes stand for 10 minutes in the water bath.
Serve warm, cold, or at room temperature.