Pluck the marzipan finely. Mix 250 g flour and 125 g sugar. Add 150 g butter in flakes, marzipan and almonds and quickly work into crumbles with the whisk of the hand mixer. Put the crumbles in a cool place.
Pour mandarins into a sieve and drain.
Mix 250 g flour and baking powder.Mix 150 g butter, 200 g sugar, oat flakes, buttermilk and vanillin sugar with the whisk of the hand mixer until creamy. Stir in eggs and sour cream one after the other.
Pour the dough into a greased springform pan (24 x 24 cm) sprinkled with flour. Alternatively use a round springform pan (Ø 28). Spread the mandarins on it and press lightly. Sprinkle with crumbles. In a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see
manufacturer) bake for about 50 minutes. If necessary, cover with foil about 10 minutes before the end of the baking time.
Remove the cake, let it cool down in the mould for about 30 minutes, remove from the mould and let it cool down on a cake rack. Dust the cake with icing sugar and cut into pieces.