Wash and trim rhubarb and cut into 1 to 2cm dice. Add to a bowl with the raspberries and mix to combine. Set to one side.
In another bowl, stir creme fraiche with 125g sugar, vanilla sugar, orange zest and eggs. Sift flour, cornflour and baking powder; add to creme fraiche mixture. Using the electric hand mixer, mix briefly until smooth.
Preheat oven to 180 C / Gas 4 and adjust your oven rack to the second lowest position. Line a springform tin with greaseproof paper. Add butter and place the tin in the oven to melt the butter, about 5 minutes.
Tilt the tin back and forth so the butter is evenly distributed. Sprinkle with 4 heaped tablespoons sugar.
Add the rhubarb raspberry mixture in an even layer.
Pour the batter over it.
Bake for 45 minutes, then turn off the oven and leave the cake for 5 more minutes.
Remove from the oven and let cool in the tin for 10 minutes, then loosen the side with a knife and gently remove the springform rim.
Place a cake plate on top and turn the cake upside down. Carefully remove the paper. Let cool completely and serve.