Grease a box mould (approx. 30 cm long; 2 3/4 l capacity) and dust with flour. Peel ginger and dice very finely. Peel, quarter and core apples. Finely dice the fruit flesh and mix immediately with lemon juice.
For the batter, whisk butter, sugar, vanilla sugar and 1 pinch of salt until creamy. Stir in the eggs one by one. Mix flour and baking powder and stir in alternately with 150 ml milk.
Cut the dough in half. Stir 2 tablespoons of cocoa, 4-5 tablespoons of milk and ginger into one half. Fold apples under the other half of the dough. First fill the light, then the dark dough into the mould. Pull through spirally with a fork - this creates the marble effect.
Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for 50-60 minutes.
Let the cake cool in the mould for about 20 minutes. Then turn out of the mould and let it cool completely.
Chop the couverture and melt in a hot water bath. Cover the cake with it and let it dry. Dust with 1 tablespoon of cocoa. Crème fraîche goes well with it.