Chop the marble cake in a bowl and mix together with the rum aroma, jam and mineral water using a hand mixer. Roll out the shortcrust pastry and cut out 4 round circles with a champagne glass, then bake in the oven according to the instructions on the packet.
Now shape mountains from the cake mixture and place on the cooled shortcrust pastry circles, brush with couverture and serve.
Tip: Rum balls can also be wonderfully prepared from the dough, you can shape balls of any size and roll them through grated chocolate. Shortcrust pastry is then not needed.