Whisk egg whites to stiff peak and set aside
Cream butter and sugar together for a few minutes
Add the flour and salt to the butter mix and mix for another minutes. Mix in the lemon juice, zest and yolks followed by the milk.
Fold in ⅓ of the egg whites (to loosen the mixture) then lightly fold in the rest of the whites.
Pour the mix into a baking dish and bake in a water bath of boiled water (water must come halfway up the baking dish) for 35- 40 minutes (or until golden on top) at 180C/375F. Top must be set and firm - not runny
