Preheat oven to Cream the butter and sugar well. Add the egg yolks one by one, then add the flour. Add the lemon zest and juice, followed by the milk, and mix well. In a separate bowl, whisk the egg whites until stiff. Fold gently into the lemon mixture until incorporated. Pour into a 9 inch springform pan or a deep cake pan. Bake for 40 minutes, or until the pudding is very lightly browned and only jiggles slightly when shaken gently. Sprinkle with powdered sugar and serve warm with freshly whipped cream. Refrigerate any leftovers.