Gently, stirring constantly, add butter, water and lemon juice. Boil the caramel until completely dissolved.
Peel the rhubarb and cut into pieces.
Add to the caramel. Add vanilla sugar. Cook everything together for a few minutes. Rhubarb should remain untouched.
Strain the caramel rhubarb through a sieve. Flip it on a sieve so that the caramel completely glasses and the rhubarb cools down.
Beat butter at room temperature with sugar.
Separately, whisk the whites with a pinch of salt.
Carefully add the whites to the yolk mass.
Add flour with baking powder and milk in several batches. Mix it.
Grease a 20×20 baking dish with butter and sprinkle with flour.
Lay out the dough. Align it.
Lay out the rhubarb.
Bake in a preheated oven at 160g for 30 minutes.
Pour caramel over the top of the cake and bake for another 15 minutes.