Wash the blueberries, drain in a sieve and sort. Put 150 g blueberries, 1 tbsp. water and lemon juice in a pot and simmer for 1-2 minutes until the berries burst open easily. Cream fat, sugar, salt and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Mix flour and baking powder and stir in alternately with the milk.
Halve the dough. Stir the smoothie into one half of the dough. Remove blueberries from the stock and drain. Remove approx. 2 tbsp. juice. Crush remaining berries and juice with a fork and stir into the second half of the dough. Pour light-coloured dough into a greased, flour-spread ring cake tin (2.2 litres). Spread blueberry dough on top and marble with a fork. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 50 minutes. Then take out of the oven, place on a cake rack and let it cool down for about 50 minutes. Turn out of the tin and let it cool down in the mould. Sift icing sugar into a bowl.
Pour light-coloured dough into a greased, flour-spread ring cake tin (2.2 litres). Spread blueberry dough on top and marble with a fork. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 50 minutes. Then take out of the oven, place on a cake rack and let it cool down for about 50 minutes. Turn out of the tin and let it cool down in the mould. Sift icing sugar into a bowl. Add blueberry juice and egg white and mix to a smooth glaze. Pour the icing over the cake, decorate with the remaining blueberries and leave to dry for approx. 30 minutes
waiting time approx. 3 hours