Prepare the pudding according to the instructions on the packet, let it cool a little and stir in the cremé fraíche.
Preheat the oven to approx. 200 degrees (convection 175 degrees).
Melt the butter. Mix the flour, sugar, vanilla sugar and salt. Add the butter and knead everything to make crumbles. Spread about 3/4 of the crumble on a greased baking sheet and press firmly.
Spread the pudding mixture on top, cover with the cleaned and cut rhubarb, sprinkle with the remaining crumble and bake on the middle rack for approx. 45 minutes.
Tip:
Also tastes good with cherries, plums or other fruits.