Cream butter, salt, 200 g sugar and vanillin sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Mix flour, starch and baking powder and stir into the fat-egg-cream alternately with the milk. Grease a ring form (24 cm Ø, 2 litres capacity) well and sprinkle with breadcrumbs. Pour in the dough and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 35-40 minutes. Let cool slightly on a cake rack. Turn out of the tin and let it cool down completely
Defrost frozen blueberries, wash and sort fresh berries. Soak gelatine in cold water. Puree 500 g blueberries, set aside 200 g puree and other berries. Mix the remaining puree with 75 g sugar and sour cream. Squeeze gelatine well, dissolve. Stir a little sour cream into the gelatine, then stir everything into the rest of the cream. Cool until the cream begins to gel
Whip the cream until stiff, fold into the cream and chill again until the cream is almost gelled. Cut the cooled base 2 times horizontally. Stir the cream once more and put it into a piping bag with a perforated spout. Pour a good 1/3 of the cream onto the bottom cake base and place the middle cake base on top. Spray a good 1/3 of the cream onto the base and cover it with the top base, press down gently. Spray the rest of the cream onto the cake and chill for about 2 hours.
Put the mashed potatoes in a pot and bring to the boil. Stir the starch with 3 tbsp. water until smooth. Stir the starch into the puree and let it simmer for about 1 minute while stirring. Remove from the heat and allow to cool slightly. Carefully fold in the remaining berries. Let it cool down. Spread the thickened blueberries with a teaspoon on the wreath. Dust with icing sugar and chill until serving
waiting time approx. 4 hours