Let the berries thaw. For the crumbles, grate marzipan coarsely. Knead with oat flakes, starch, 50 g sugar and 50 g cold butter in pieces to form coarse crumbles. Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Grease a box mould (approx. 11 x 25 cm; 1 3⁄4 l content). Place a strip of baking paper (approx. 20 x 40 cm) in the baking tin so that the narrow sides remain free.
For the dough, whisk 200 g soft butter and 120 g sugar with the whisks of the mixer for about 5 minutes until creamy. Stir in the eggs one after the other. Mix rice flour, baking powder, vanilla sugar and 1 pinch of salt. Sieve portions onto the egg-sugar-cream and stir in briefly.
Peel, quarter, core and cut the apple into small cubes. Fold into the dough with berries. Fill the dough into the tin, spread crumbles on top and bake in the hot oven on the second rack from the bottom for 50-60 minutes, covering with baking paper after 15-20 minutes if necessary.
Take the cake out of the oven. First cool in the mould for approx. 30 minutes, then carefully lift out and let it cool down. Whipped cream tastes good with it.