For the batter: whiks butter with sugar, egg and flour until smooth. Butter springform pan and spread with batter, smooth.
For the topping: rinse and clean blueberries, pat dry and arrange on top of cake.
Beat eggs with sugar, vanilla sugar, cream and creme fraiche and pour over blueberries.
Bake cake in preheated oven at 180°C (approximately 350°F) for about 45 minutes. Remove from the oven and serve warm or cold.