Boil the water with the butter in a small saucepan. Remove the pot from the oven. Mix the flour with the cornstarch and add all at once to the hot liquid. Mix everything into a smooth dough, then heat (burn off) for about a minute, stirring constantly.
Put the batter in a mixing bowl. Work 2 eggs one after the other with a mixer (dough hook) on the highest setting under the dough. First stir the baking powder into the cold batter.
Using a piping bag with a large star nozzle, place 14 piles of dough on the baking sheet and bake in a preheated oven at 200 ° C top / bottom heat for about 20 minutes. Do not open the oven door during the baking time, otherwise the baked goods will collapse.
Immediately after baking, cut off a lid from each cream puff and let the pastry cool on a wire rack. Beat the cream with the cream stiffener, powdered sugar and vanilla sugar and use it to fill the cold cream puffs.