Preheat oven. Grease a 26 springform pan or just line the base with baking paper.
Beat the butter, sugar and vanilla sugar until frothy. Stir in eggs. Fold in the flour and baking powder. Melt the chocolate mint bars in a water bath and pull them under half of the batter. Place the dark and light dough either side by side, on top of each other (like a marble cake) or in dots (like a zebra cake) in the mold.
Bake in the oven at 175 ° (convection 160 °) on the middle rack for 50-60 minutes. Let cool slightly and remove from the mold.
Melt both couvertures separately in a water bath. For a better result, add 10 g of coconut oil to each. Place the light and dark couverture side by side on one half of the cake and pull together with the help of a wooden stick.