Preheat the oven to 350 degrees F. Lightly butter an 8 inch x 8 inch baking dish. Place the 1/8 cup butter and the sugar in a bowl and beat until well combined. Beat in the egg yolks. Mix in the flour, and then mix in the milk, lemon zest and lemon juice (batter will be very thin). In another bowl, beat the egg whites until soft peaks form. Whisk the beaten egg whites into the batter. Spoon the batter into the baking dish. Bake for 45 to 50 minutes, or until the top is set and light golden. Let stand for 15 minutes before serving. If desired, top portions of lemon pudding cake with whipped cream and garnish with a mint sprig.