Boil the milk with sugar and a piece of vanilla or with 1 dessertspoonful of vanilla essence. Smooth the cornflour with a little cold milk, and thicken the milk with it. Whip the whites of the eggs to a very stiff froth with 1 spoonful of castor sugar, and drop spoonfuls of the froth into the boiling milk. Allow to boil until the balls are well set, turning them over that both sides may get done. Lift the balls out with a slice, and place them in a glass dish. Beat up the yolks of the eggs, stir them carefully in the hot milk; let the custard cool, and pour it into the glass dish, but not over the snowballs, which should remain white.