Melt butter. Slowly add flour, and blend it in with a wooden spoon. Put the sugar into a pot with the milk and heat to scalding but do not boil. Remove from the heat and pour into the flour; stir quickly to make a thick cream (this may be returned to the heat for a few minutes if necessary). Partially cool. Beat the egg yolks with the vanilla until very thick. Beat the egg whites until stiff but not dry. Add yolks to cream mixture; stir well. Fold in the beaten whites. Pour into buttered glass baking dish or into 6 or 8 individual buttered remekins. Bake immediately in preheated 300 F. oven for abt. 20 min. Serve hot, sprinkled with powdered sugar. VARIATION: Omit vanilla and replace with 1 tb. grated orange or tangerine peel (Soufle Portocaliou E Mandnarinou).