Preheat the oven to 165°C. Prepare a cake tin (26 cm diameter or 2x 18 cm diameter) by greasing the sides and the bottom of the tin.
Cream the butter with mascarpone cheese for 1-2 minutes on medium speed. Add sugars and mix for 3 minutes until smooth.
Add eggs and an egg white - one egg at a time while still mixing. Mix until it's well combined.
In another bowl whisk together flour, baking powder, salt, and cornstarch.
Start by adding ⅓ dry ingredients, mix until the flour disappears, and 1 ½ tablespoon of jam - repeat the process until you add dry ingredients the last.
Bake for 35-45 minutes or until the toothpick comes out clean. Take the cake out of the oven and let cool completely.
Once the cake layer is completely cool, whip the cream with vanilla sugar and cream stiffeners. Add cream on top of the cake layer, sprinkle with fresh raspberries, and pour some raspberry jam on top as well. Keep in the fridge for the best taste possible.