Add the raisins to a small bowl and pour the 1/2 cup dark rum over them. Stir, then cover and let soak for at least 30 minutes, or up to overnight in the refrigerator, until the raisins are plump and soft. For a quicker soak, microwave the raisins and rum together for 20–30 seconds to warm them slightly, then let stand.

Line an 8×8‑inch (20×20 cm) baking pan with parchment paper, leaving an overhang on two sides to use as a sling. Lightly butter the parchment or spray with nonstick cooking spray to prevent sticking.

Once the raisins are plump, drain them over a small bowl to catch the soaking rum. Pat the raisins dry with paper towels so they don’t water down the fudge. Reserve 1–2 tablespoons of the soaking rum to add to the fudge for extra flavor, if desired.

Add the chocolate chips, sweetened condensed milk, and butter to a medium heavy‑bottomed saucepan. Cook over low heat, stirring constantly with a silicone spatula or wooden spoon, until the mixture is fully melted, glossy, and smooth. Keep the heat low to avoid scorching or seizing the chocolate. (Microwave method: Combine the chocolate, sweetened condensed milk, and butter in a microwave‑safe bowl and heat in 20–30 second bursts, stirring well after each, until smooth.)

Remove the saucepan from the heat. Stir in the vanilla extract and fine sea salt until fully incorporated. If you want a stronger rum flavor, whisk in 1–2 tablespoons of the reserved soaking rum, to taste. The warm mixture should taste slightly stronger than you want the finished fudge to be, as the flavor will mellow while chilling.

Gently fold the drained, patted‑dry raisins into the warm chocolate mixture until evenly distributed. If you like additional texture, fold in up to 1/2–3/4 cup chopped toasted nuts at this point.

Pour the fudge mixture into the prepared pan and use a spatula to spread it evenly into the corners and smooth the top. If desired, sprinkle flaky sea salt, extra chopped rum‑soaked raisins, nuts, or drizzle melted white chocolate over the surface, gently pressing any solid toppings so they adhere as the fudge sets.

Refrigerate the pan of fudge for at least 2 hours, or until completely firm to the touch. For the cleanest cuts and best texture, chill 3–4 hours or overnight. If desired, let the pan stand at room temperature for 20–30 minutes before refrigerating to reduce condensation on the surface.

Use the parchment sling to lift the chilled fudge out of the pan and place it on a cutting board. With a sharp knife warmed under hot water and wiped dry, cut into about 1‑inch squares, wiping the blade clean between cuts for neat edges. Serve chilled or at cool room temperature; the fudge will soften slightly as it warms.

For an alcohol‑free version, soak the raisins in hot water, apple juice, or orange juice instead of rum, then drain and pat dry. Skip the added soaking rum and instead stir 1–2 teaspoons of rum extract into the warm chocolate mixture along with the vanilla and salt, tasting and adjusting as needed.