In a small saucepan, combine the dark raisins, dark spiced rum, ground cinnamon, and ground nutmeg over medium heat.

Bring the mixture to a gentle simmer, then reduce the heat to low and let it cook for about 5 to 7 minutes, or until the raisins are plump and have absorbed most of the liquid. Remove from the heat and let the mixture cool completely to room temperature.

In a large chilled mixing bowl, whip the cold heavy whipping cream using a hand mixer or stand mixer on medium-high speed until stiff peaks form, which should take about 3 to 4 minutes.

In a separate medium bowl, whisk together the sweetened condensed milk, pure vanilla extract, fine sea salt, and any remaining rum liquid from the cooled raisin mixture until smooth and combined.

Gently fold the cooled plump raisins into the condensed milk mixture using a rubber spatula until they are evenly distributed.

Gradually fold the condensed milk and raisin mixture into the whipped cream, working in batches and being careful not to deflate the whipped cream, until the mixture is fully incorporated and uniform.

Transfer the ice cream mixture into a 9x5-inch loaf pan or airtight container, smoothing the top with a spatula, then cover tightly with plastic wrap and freeze for at least 6 hours, or until firm, before scooping and serving.