For the dough, grease a springform pan (28 cm Ø) and dust with flour. Clean, wash, dab dry and cut into cubes. Separate 4 eggs, chill the egg white. Mix 200 g butter, 100 g sugar, 1 packet vanilla sugar and 1 pinch of salt with the whisks of the mixer for about 5 minutes until creamy. Stir in 4 egg yolks and 3 eggs one after the other. Mix 200 g flour and baking powder and stir in briefly with the cream.
Put the dough into the mould and smooth it down. Mix 250 g rhubarb and 1 tablespoon sugar and spread on the dough. Pre-bake the cake in a preheated oven (electric oven: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 20 minutes.
For the meringue, beat 4 egg whites until stiff, adding 200 g sugar and 1 packet of vanilla sugar. Mix 375 g rhubarb with starch and fold into the meringue. Remove the cake from the oven. Spread the rhubarb meringue loosely on the meringue, leaving about 2 cm free around the edges.
Bake the cake at the same temperature for about 35 minutes. Place on a cake rack. Carefully remove the cake from the edge with a sharp knife, remove the edge of the mould and let the cake cool down. Dust with icing sugar. Whipped cream tastes good with it.