Peel and cut the apples into cubes
Cook the apples with sugar and water until tender (about 10 minutes) in a saucepan without a lid
Mash the apples by blending them in a blender
Add the apples to a bowl with 150 g butter and stir until the butter has melted
Mix flour and baking powder in a different bowl
Pour flour into the applesauce and stir till combined
Rinse and cut the rhubarb into cubes / pieces
Lubricate the spring form with a bit of vegetable butter or oil
Pour the dough into the springform pan. Sprinkle with rhubarb on top
Bake the cake in the oven at 190° C for 35-40 min