1. Adjust oven rack to center position and heat oven to 325 degrees. Lightly butter an 8″ square baking pan. Bring several quarts of water to boil for water bath. Mix flour and cornstarch together in a bowl, set aside.

2. Cream the butter, 1 c. sugar and lemon zest until light and fluffy, about 2 minutes. Then, beat in the egg yolks, then flour mixture, mixing until smooth. Slowly mix in the lemon juice, then stir in milk.

3. Set the custard mixture aside and beat the egg whites until they hold soft peaks. Then, add the remaining ¼ c. of sugar and beat egg whites to stiff, moist peaks. Gently stir a dollop of the egg whites into the custard. Then, working slowly, fold the remaining egg whites in bit by bit until they are all incorporated, but not over beaten.

4. Pour the batter into the baking pan and fill the hot water bath until it comes halfway up the side of the pan. Bake until pudding cake center is set and springs back when gently touched, about 1 hour. Remove roasting pan from oven and let pan cool for 1 hour before serving.