Wash, pit, and quarter the plums. Watch the video for getting my secret plum-cutting hack! Then keep them cool.
Combine all the ingredients for the dough in a bowl and mix with a hand mixer until smooth.
Grease a half baking sheet with a rim and spread the dough evenly on it.
Sprinkle the dough with breadcrumbs.
Spread the prepared plums evenly over the dough.
Mix the brown sugar, vanilla sugar, and cinnamon well, then sprinkle evenly over the plums.
Preheat the oven to 350°F (180°C).
Combine the ingredients for the streusel in a small bowl and knead into a crumbly dough with your fingers.
Spread the streusel evenly over the plums.
Bake the cake on the middle rack for about 50 minutes until the streusel is golden brown and the cake is baked through.
