Pre-heat the oven to 350°F. and set a rack in the middle of the oven.
Place butter in a medium mixing bowl; gradually add sugar as you mash the mixture with a wooden spoon until it looks like damp sand.
Mix egg yolks into sugar mixture; beat in the flour; add lemon zest and juice; then whisk in the milk.
In a separate clean large bowl, beat egg whites to stiff peaks; fold whites, by hand, gently into batter.
Pour mixture into a 5-cup ceramic, glass baking dish or Pyrex pie plate.
Place in the pre-heated oven and bake in the middle of the oven for about 40 minutes, or until mixture is just set and top is golden brown.
Alternatively, bake in individual ramekins or custard cups for about 20 minutes.
Serve warm, dusted with confectioners’ sugar and softly whipped cream alongside.
