Beat the margarine until creamy then gradually beat in the sugar, vanilla sugar and eggs. Sift the flour, cornstarch and baking powder and stir into the butter-egg mixture until just combined. If necessary, stir in the milk.
Rinse the blueberries and pat dry. Line a baking sheet with parchment paper, fill with the batter, smooth the surface, sprinkle with blueberries and bake in a preheated oven at 175°C (approximately 350°F) for about 40 minutes.