Make a kneading dough from flour, baking powder, butter, sugar and 1 egg.
Clean the rhubarb and cut into small pieces, add some sugar. Cook the pudding with the milk according to the instructions. Stir in sour cream and rhubarb.
Roll out the dough in a 26 cm springform pan, pulling the edge up a little. Spread the pudding and rhubarb mixture on the dough.
Bake for about 45 minutes at 190 ° C fan oven.
Then whip the egg white with 1 tablespoon of sugar until stiff, spread it on the cake and bake for another 5 minutes at 190 ° C fan oven.