In a large saucepan, combine the prunes, water, rum, and ground cardamom. Bring to a gentle simmer over medium heat and cook for approximately 15 minutes, or until the prunes have softened completely. Remove from heat and stir in the vanilla extract and punsch, if using. Using an immersion blender (or food processor), blend the mixture until smooth. Transfer the filling to a bowl, cover, and allow it to cool to room temperature.

Preheat the oven to 180°C (350°F). Line two large baking trays with parchment paper and lightly brush the parchment with a small amount of neutral oil or use a non-stick liner.

In a mixing bowl, cream the softened butter and caster sugar together until pale and fluffy. Add the egg, vanilla extract, salt, cardamom, and cinnamon. Beat until well incorporated. Add the flour and baking powder gradually, stirring with a wooden spoon until the dough becomes too stiff. Knead gently by hand until a uniform dough forms.

Divide the dough into seven equal portions. Roll each portion into a thin layer using a rolling pin, shaping as desired (typically squares or circles). Use a template if uniformity is preferred. Place each rolled sheet onto the prepared baking trays and bake for approximately 12 minutes, or until the edges begin to turn golden. Monitor baking closely, as timing may vary by oven. Cool each layer on the tray for 5 minutes before transferring to a wire rack to cool completely.

Once all layers have cooled, begin assembling the cake by spreading an even layer of the cooled prune filling between each baked shortbread sheet. Stack carefully to maintain alignment. Do not spread filling on the top layer.

Wrap the assembled cake tightly in cling film or plastic wrap. Store in a cool, dry place (not the refrigerator) for a minimum of 3 days. This resting period allows the flavors to meld and the layers to soften.

When ready to serve, trim the edges for a cleaner appearance if desired. Slice thinly using a sharp knife and serve at room temperature.