In a small saucepan over medium-low heat, combine the dark raisins, dark rum, ground cinnamon, and ground nutmeg. Simmer gently for about 8 to 10 minutes until the raisins absorb most of the liquid and become plump and tender. Remove from heat and let the mixture cool completely to room temperature before using.

In a large mixing bowl, pour in the cold heavy whipping cream. Using an electric hand mixer or stand mixer fitted with a whisk attachment, beat on medium-high speed until stiff peaks form, which should take about 3 to 4 minutes.

In a separate medium bowl, stir together the sweetened condensed milk, pure vanilla extract, and fine sea salt until fully combined and smooth. Gently fold in the cooled rum-infused raisin mixture, ensuring any remaining liquid from the saucepan is incorporated.

Add a third of the whipped cream to the condensed milk and raisin mixture, folding gently with a silicone spatula to lighten the base. Gradually fold in the remaining whipped cream in two batches, being careful not to deflate the mixture so the ice cream remains light and airy.

Transfer the combined ice cream mixture into a standard 9x5-inch metal loaf pan, smoothing the top with your spatula. Cover the pan tightly with plastic wrap directly touching the surface of the cream to prevent ice crystals from forming, then freeze for at least 6 hours or overnight.

When ready to serve, let the ice cream sit on the counter for about 5 to 10 minutes to soften slightly. Scoop into bowls or waffle cones, and optionally garnish with a light dusting of cinnamon or an extra drizzle of dark rum.