Select the blueberries, wash and drain. Finely puree 100 g berries and 1 packet of vanilla sugar. Grease a small cake tin (18 cm Ø) and dust with flour. Whisk butter, salt and sugar with the whisk of the hand mixer until white-creamy. Stir in the eggs one after the other. Mix flour and baking powder. Alternately stir in with buttermilk. Spread the dough evenly in the form and smooth it down.
Pour the berry puree onto the dough, pull it through spirally with a fork. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes. Remove from the oven, leave to cool in the mould for approx. 10 minutes, then turn out onto a cake rack. Let it cool down.
Mix remaining blueberries with jam. Beat the sour cream and 1 sachet of vanilla sugar until stiff. Fold in about half of the blueberries. Fill into the deepening of the ring cake. Spread the rest of the blueberries on top.