Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease the box form (25 cm long; 1 1⁄4 l capacity) and dust with flour. Cream 200 g butter, 150 g sugar and 1 pinch of salt with the whisks of the mixer.
Stir in 4 eggs one after the other. Stir in plum jam. Mix 200 g flour, cocoa and 2 tsp. baking powder and stir in briefly.
Fill the dough into the mould and smooth it down. Bake in a hot oven for about 50 minutes. If necessary, cover with aluminium foil after 35-40 minutes. Take out and let cool on a cake rack for about 15 minutes.
Then carefully turn out of the mould and let it cool down. Clean the box mould, grease it again and dust it with flour.
Cut slices (approx. 1 1⁄2 cm thick) from the chocolate cake. Cut out a total of 16 stars (approx. 5 cm Ø) from the slices, but do not use the ends of the cake.
Coarsely grate the marzipan. Cream 200 g butter, 100 g sugar, 1 pinch of salt and marzipan with the whisks of the mixer. Stir in 4 eggs one after the other. Mix 200 g flour and 2 tsp. baking powder. Stir in flour mix and milk.
Pour some marzipan dough into the box mould. Place the chocolate stars inside. Fill in the rest of the dough. Bake in a hot oven at the same temperature for about 50 minutes. If necessary, cover with aluminium foil about 15 minutes before the end of the baking time.
Take out the cake, let it cool down a little in the mould. Then remove from the mould and let it cool down on a cake rack. Dust with icing sugar.