Grate 50 g of the marzipan. In your stand mixer, cream butter with grated marzipan for about 3 minutes. Add the powdered sugar, a couple of drops of bitter almond aroma, and the salt. Whisk for about 2 minutes. Add the egg and mix until combined.

Combine flour, cornstarch and poppy seeds. Add dry mixture to the creamed mixture and stir until combined. Dump the batter on a large piece of cling film. Cover with another piece of cling film and shape dough into a disc. Let the cookie dough rest for about 2 hours.

Preheat the oven to 350 F and line 2 cookie sheets with parchment paper. Roll out the dough to about 3-4 mm and cut out using a round cookie cutter. Bake for about 8 minutes. Let cool completely.

Stir rum into the plum jam until smooth. Roll out marzipan on a powdered sugar-dusted surface and cut out rounds using the same cookie cutter, you used for the cookie dough.  Spread a bit of jam on one cookie, put a marzipan-round on top, spread a tiny bit more plum jam on the marzipan and top with another cookie. Press together very gently and store in cookie tins.