Cream butter, 200 g sugar, salt and vanillin sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Mix 200 g flour, buckwheat flour and baking powder and stir in alternately with the milk.
Put half of the dough into a greased springform pan (26 cm Ø) sprinkled with flour and spread. Mix the remaining dough with beetroot juice and 2 tbsp. flour. Fill the dough into the springform pan as well and swirl a little with a fork.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 40 minutes, remove from oven and let cool on a cake rack.
Soak gelatine in cold water. Whip the cream until stiff, while pouring 75 g of sugar. Mix 250 g cranberries with about half of the cream. Squeeze the gelatine and melt it in a pot.
Add 5 tablespoons cranberry cream to the gelatine, stir the gelatine into the cranberry cream. Fold in the remaining cream.
Carefully remove the base from the springform pan and place on a cake plate. Cut the base in half horizontally. Spread the bottom layer with about 1/3 of the cranberry cream. Put the lid on and spread the rest of the cream all around the cake.
With the help of a spoon, make grooves in the cream. Chill the cake for at least 2 hours. Decorate with 2 tablespoons of cranberries.