Clean and wash the rhubarb and cut into small pieces. Mix with 350 g sugar and refrigerate overnight.
The next day, drain the rhubarb, collect the juice and, if necessary, make up to 750 ml with water. Grease the drip pan. Mix the butter, 200 g sugar and salt until creamy. Gradually stir in 4 eggs. Mix the flour and baking powder, stir in portions. Spread the batter on the drip pan.
Mix the pudding powder with 12 tablespoon rhubarb juice until smooth. Bring the rest of the juice to the boil, stir in the mixed pudding powder and briefly bring to the boil again. Let the pudding cool down. Separate 5 eggs. Beat 5 egg whites with a pinch of salt until stiff. Halve the pudding. Carefully fold the egg whites under half of the pudding. Gradually add the second half of the pudding to the 5 egg yolks while stirring. Spread the egg yolk pudding on the batter.
Spread the rhubarb pieces evenly on top. Spread the egg pudding on the rhubarb. Bake the cake in a preheated oven (electric stove 175 degrees, convection 150 degrees, gas level 2) for about 45 minutes. Cover with parchment paper after 30 minutes.
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