Preheat oven to 320°F. Melt the butter in a pot and set aside. Heat the milk along with the vanilla bean.
Cream the egg yolks with the powdered sugar, melted butter and water. Stir in the flour and milk. Gently add the 4 beaten egg whites into the mixture. Pour batter into a baking dish covered with parchment paper and bake for 40 minutes.