Add the baking powder to the flour in a bowl and mix the two together. Now add the soft butter cut into pieces, the sugar, the vanilla sugar and the eggs and knead everything with your hands to a crumble dough. Alternatively, you can use the dough hook of the mixer.
Spread two thirds of the crumble dough on a baking tray lined with baking paper and press it briefly flat with your hands to form a dough sheet.
Leave the tray and the remaining dough to rest in the meantime, preferably in a cool place. In the meantime, wash and clean the strawberries and place them in a large bowl. Cut the larger strawberries in half.
Wash and clean the rhubarb, cut into pieces of about three centimeters and add to the strawberries. Make a mixture of 4 tablespoons of sugar and 2 tablespoons of custard powder, pour it over the fruit and mix everything carefully – without crushing the strawberries. Put the fruit on the pastry base and spread it evenly.
Then add the remaining crumble on top.
Bake the cake at 180 degrees, convection oven 160 degrees, for 40 – 45 minutes. Let the baked cake cool and serve with whipped cream.