Preheat your oven to 325°F (163°C). Grease and flour a 10-inch bundt pan or two 9-inch round cake pans.

In a small bowl, combine the raisins and dark rum. Let them soak for at least 1 hour, or overnight for a richer flavor.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

In another bowl, whisk together the flour, baking powder, cinnamon, nutmeg, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with the milk and vanilla extract. Mix until just combined.

Fold in the soaked raisins (with any remaining rum) and chopped walnuts if using.

Pour the batter into the prepared bundt pan or divide it evenly between the round cake pans.

Bake in the preheated oven for 50-60 minutes for the bundt cake or 30-35 minutes for the round cakes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.