Chop the couverture and melt in a water bath over low heat. Grease the bottom of the springform pan.
Preheat the oven: Top / bottom heat: 170 ° C.
Separate the eggs and beat the egg whites with salt with a mixer until very stiff.
Mix the butter in a mixing bowl with the mixer, first on the lowest, then on the highest level until frothy.
Gradually stir in the sugar and vanilla sugar until you get a bond.
Slowly stir in the egg yolks, add the coffee powder and melted couverture. Briefly work in the flour and then fold in the egg whites.
Smooth out the dough in the pan and place on the wire rack in the oven. Insert: lower third baking time 32 min.
Place the cake on a wire rack, let it cool down a little, then loosen and remove the cake.
Can be served cold or warm.