2.. In a mixing bowl, cream the butter or margarine well with an electric mixer. Add the sugar substitute and sugar and beat well.
3.. With an egg separator, separate the eggs; save the egg whites in another mixing bowl. Add the yolks to the butter mixture one by one. Gradually add the lemon juice and lemon zest. The batter will look like lemon frosting at this point. Slowly add the milk to make a thin batter.
Set aside.
4.. In a separate bowl, beat the egg whites with an electric mixer until they are stiff. Gently fold the egg whites into the lemon mixture. Pour the batter into the prepared pie plate.
5.. Place the pie in the preheated oven and bake 30 to 40 minutes. Remove and allow the pudding cake to cool.