Beat egg whites in a large bowl until stiff, gradually add powdered sugar. Beat until mixture is white and makes peaks.
Whisk egg yolks with vanilla seeds and fold in 3 tablespoons of egg whites. Gradually fold remaining egg whites. Combine flour and cornstarch, sift over egg mixture and fold in gently.
Heat milk and dissolve vanilla sugar in it.
Butter oval gratin dish and pour milk into it.
With a spatula, separate 4-5 dumplings from the mixture and arrange in a dish.
Bake in preheated oven at 200°C (approximately 400°F) for about 10-12 minutes or until golden brown. Remove from the oven and sprinkle with powdered sugar. Serve.