Beat the eggs with the sugar until the latter dissolves. Add the milk, jam, the baking soda extinguished in 1 tbsp vinegar and flour mixed with baking powder.
Pour the mixture into a small oven dish laid out with baking paper and bake in a preheated 392°F (200 °C) oven until ready. Cut the baked layer in 2 horizontally, leave to cool.
Mix the sour cream with the powdered sugar and vanilla sugar, beat until homogeneous.
Assemble the cake, smearing it with cream in the middle, on the top and sides.