Preheat the oven to 175 degrees Celsius (347 degrees Fahrenheit).
Stir butter, sugar, and vanilla sugar until soft and creamy.
Beat the eggs in one at a time – stir for a minute between each.
Add flour, baking powder, and crème fraiche to the batter and fold well together. Place the batter in a baking dish (approx. 20x20cm), greased or lined with parchment paper.
Slice the plums, toss them in the sugar, and distribute them over the batter.
Crumble
Coarsely chop nuts and almonds and mix with the other ingredients – until you get a crumbly texture.
Spread it over the plums and bake the cake in the oven for about 45 minutes.