Mix the peeled rhubarb with 2 tablespoons of sugar. Mix the flour with the baking powder, lemon zest and a pinch of salt.
For the dough, first separate the eggs, then pour the egg yolks and egg whites into two different mixing bowls. Beat the egg whites with an electric hand mixer to form firm egg whites. Mix the egg yolks with the soft butter and sugar until frothy. Sift the flour mixture over it and stir in. Finally, fold the egg whites into the dough with a spoon.
Pour the batter into a prepared cake tin and smooth it out. Lay the rhubarb pieces evenly on top. Bake in the preheated oven at 200 ° C top / bottom heat on the middle rack for approx. 35 - 45 minutes.