Beat the eggs with the butter, sugar and vanilla sugar until frothy. Then stir in the flour and baking powder. Put half of the dough in the baking pan. Roll out the marzipan thinly (e.g. between two plastic sheets) and cut it out to fit the cake pan. Place the rolled out marzipan on the dough and do not spread the jam too thinly. Mix the cocoa powder with the rest of the batter and pour onto the jam layer. Bake at 175 degrees for 70 minutes (stick test). Melt the couverture in a water bath and spread it on the cake.
You can also use silicone baking molds, but normal baking molds are better. The silicone expands slightly, causing the dark dough to flow down past the marzipan.
Anything can be used as a jam, a slightly sour cherry jam goes well with the sweet cake.