Grease a fat pan (approx. 32 x 39 cm; approx. 3.5 cm deep) and dust with flour. Clean and wash the rhubarb, cut into thick slices and weigh 1.5 kg. Mix with 225 g sugar and 1 packet of vanillin sugar in a pot, let stand for about 30 minutes.
Mix 5 tablespoons of milk and pudding powder. Boil up the rest of the milk and 40 g of sugar. Stir in the pudding powder and simmer for about 1 minute while stirring. Let it cool down, stirring more often.
Add lemon zest to the rhubarb, bring to the boil and simmer for about 5 minutes. Stir frequently. Mix 130 g starch and 200 ml water. Stir into the rhubarb and simmer for 1-2 minutes. Let it cool down, stirring more often.
Cream fat, 200 g sugar, 1 packet of vanilla sugar and 1 pinch of salt for about 5 minutes. Stir in eggs one by one alternating with 50 g starch. Mix flour, 50 g starch and baking powder, stir in portions briefly.
Spread on the fat pan.
Mix the pudding briefly with the hand mixer. Spread on the sponge mixture. Spread the rhubarb groats on top. Bake in the preheated oven (electric oven: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 20 minutes.
Sprinkle with almonds and 2 tablespoons of sugar and continue to bake for about 30 minutes. Let it cool down. Dust with icing sugar.