Preheat the oven to 200°C (approximately 400°F). Beat the egg whites in a large bowl until stiff, then gradually sprinkle in half of the powdered sugar. Beat again until a shiny, hard meringue forms.
Beat the egg yolks with the remaining powdered sugar and the vanilla seeds. Add 3 tablespoons of meringue and stir, then add the remaining meringue. Carefully sift in the flour and cornstarch.
Heat the milk in a sauce pan and dissolve the vanilla sugar into it.
Grease an oval gratin dish with butter, then pour the milk into it.
With a spatula, scoop four or five large drops of batter side by side into the baking dish.
Bake on the middle rack until golden brown, about 10-12 minutes. Serve dusted with powdered sugar.