Beat the egg whites with sugar and vanilla sugar into a thick, white fluffy foam.
3 egg whites, 200 g sugar, 1 tablespoon vanilla sugar
Sift the flour together with the baking powder through a sieve.
500 g wheat flour, 2 teaspoons baking powder
Add the egg yolks to the beaten egg whites and mix briefly.
3 egg yolks
Pour in the oil in a thin stream, mixing constantly.
120 ml oil
Add the buttermilk and mix.
250 ml buttermilk
Add the flour and mix only with a spoon.
Transfer the batter to a baking pan of approximately 20/30 cm.
Arrange the chopped rhubarb on top.
about 500 g rhubarb
Mix the crumble ingredients together - crush with your fingers until lightly combined.
130 g wheat flour, 70 g butter, 60 g sugar, 1 tablespoon vanilla sugar
Sprinkle the crumble topping over the cake.
Bake at 170 degrees (convection mode) for about an hour - until a dry skewer comes out clean.
After baking, let it cool.
Uwagi
I don't sweeten cakes much, so if you like sweeter cakes, you can add more sugar, or before arranging the rhubarb on the cake, sprinkle it with sugar and mix to make it less sour.