In a large bowl, combine 200g room-temperature butter, 300g flour, 200g sugar, 1 packet vanilla sugar, the rum aroma, 3 drops bitter almond flavor, 1 pinch cardamom, 1 pinch salt, and 150g walnuts. Knead by hand for 5 min until you have a smooth dough. Wrap tightly in cling film and refrigerate for at least 1 hour.
Lightly flour your work surface. Roll out the dough to 1 cm thick. Cut out star shapes using a star-shaped cookie cutter. Line a baking sheet with baking paper. Preheat your oven to 200 °C / 390 °F (convection: 180 °C / 360 °F). Place the stars on the baking sheet and bake for 10 min until light golden. Transfer to a wire rack and let cool completely to room temperature.
Knead the marzipan until soft and pliable. Roll it out thinly between two layers of cling film. Cut out stars using the same cutter. Brush each cooled cookie thickly with currant jelly. Place a marzipan star on each cookie and press gently to adhere.
Place the couverture in a heatproof bowl set over a pot of simmering water, making sure the bowl does not touch the water. Stir occasionally until completely melted and smooth. Dip each marzipan-topped cookie into the melted couverture to coat, or brush it on. While the chocolate is still warm, press one walnut half onto the top of each star. Let dry at room temperature for about 30 min until completely set.