Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease a box mould (28 cm long; approx. 1.75 l). Cream 225 g butter, sugar, vanilla sugar and 1 pinch of salt with the whisks of the mixer.
Stir in the eggs one by one. Mix flour, cocoa and baking powder. Alternately stir in the flour mix and red fruit jelly.
Pour the dough into the tin and bake in a hot oven for 55-60 minutes. Prick in with a wooden skewer. If the dough is still very soft, continue baking for about 10 minutes. Take out the cake and loosen the edges. Leave to cool in the tin and take it out later.