For the rhubarb and vanilla custard crumble cake, first prepare the dough. To do this, mix the softened butter, sugar and vanilla sugar in a bowl. Gradually stir in the eggs.
In another bowl, mix the flour, almonds, baking powder and salt. Gradually add this mixture to the buttercream.
Stir in the grated lemon zest. Meanwhile, preheat the oven to 175 °C. Grease a baking tray or a cake pan, pour in the batter and smooth it down.
Now put the whole thing in the refrigerator.
For the filling, cook the vanilla pudding according to instructions and also put it in the fridge. Wash the rhubarb, peel it (cut off the ends and use a knife to peel the skin from one end to the other – just like asparagus) and cut it into pieces about 1 cm thick.
Pour the custard on the base and spread the rhubarb pieces evenly on top.
For the crumble, knead all the ingredients and crumble the mixture over the custard with your fingers. Put the whole thing in the oven for about 1 hour until the crumbs are lightly brown.