For the dough, beat the margarine with the sugar until frothy, stir in the egg. Fold in the flour with the baking powder, line a greased dish with breadcrumbs and pull up one edge. Pre-bake at 175 ° C (hot air) for approx. 10 minutes.
For the topping, drain the sour cherries while collecting the juice. Boil the vanilla pudding (without sugar!) With 1/2 l of cherry juice (and possibly 1/2 small glass of kirsch), fold in the sour cherries and spread on the dough.
For the crumble, beat the butter with the sugar until frothy. Knead in the flour with the hazelnuts and spread over the cherries. Bake at 175 ° C for approx. 45 minutes.
Let rest for 1 day and before serving, whip 2 cups of cream with cream stiffener until stiff, place on the cake and dust with cocoa or decorate as desired.