Preheat the oven to 350 degrees F
In a medium bowl sift together the sugar and the flour. Combine the melted butter, lemon zest, and lemon juice and add to the dry ingredients.
Divide the egg yolk from the egg white. Beat the egg yolks until well beaten and mix in the milk. Add the egg and milk mixture to the cake batter.
With clean beaters beat the egg whites until stiff peaks form. Fold the egg whites into cake batter.
Pour the batter into an 8 x 8-inch baking pan. Place the baking pan into a larger pan and add water until it is about 1 inch deep.
Place in the preheated oven and bake for 40 to 45 minutes. Serve the cake warm or chilled.