In a small bowl, combine the chopped nuts, raisins, and dark rum. Let them soak for about 30 minutes to infuse the flavors, then drain and set aside. For a smoother texture, pulse the soaked fruit in a food processor before folding it in.
In a mixing bowl, cream together the softened butter and brown sugar until light and fluffy. A stand mixer on medium speed works well here too.
Beat in the egg and molasses until well combined.
In a separate bowl, whisk together the all-purpose flour, ground allspice, cinnamon, nutmeg, cloves, baking soda, and salt.
Add the wet ingredients to the dry ingredients, mixing until a soft, sticky dough forms.
Fold in the soaked nuts and raisins (or currants), distributing them evenly through the dough.
Shape the dough into a log and wrap it in plastic wrap. Refrigerate for about 1 hour, or until the dough is firm.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Slice the chilled dough into rounds, about 1/4 to 1/2 inch thick. Place the cookie rounds on the prepared baking sheet, leaving some space between them.
Bake in the preheated oven for 12-15 minutes, or until the edges are lightly golden.
Remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Once the cookies are fully cooled, dust them with powdered sugar. Serve with whipped cream, or just as they are.