Clean and wash the rhubarb and cut it into pieces about 1.5 cm wide. Stir pudding powder and 100 ml juice until smooth. Put the rest of the juice, rhubarb and 150 g sugar in a pot. Bring to the boil, cover and simmer for about 2 minutes.
Stir the mixed pudding powder into the boiling liquid. Let simmer for about 1 minute while stirring. Remove from heat and set aside. Cream fat, 200 g sugar, vanilla sugar and salt with the whisk of the hand mixer.
Stir in 5 eggs one after the other. Mix 300 g flour, almonds and baking powder, stir briefly into the fat-egg cream with the milk. Grease the fat pan of the oven (32 x 39 cm) and dust with flour.
Put the dough on the fat pan and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 20 minutes. In the meantime, stir marzipan and 75 g sugar until creamy with the whisk of the hand mixer.
Stir in 3 eggs one after the other. Stir in 100 g flour by the spoonful. Pour into a piping bag with star-shaped spout. Spread the compote on the base and smooth it down. Spray marzipan mixture onto the cake in a grid pattern.
Bake at the same temperature for 20-25 minutes. Let it cool down and dust with icing sugar. Whipped cream tastes good with it.