Place the raisins into a bowl and pour the rum over them. Leave them to soak for at least an hour but preferably overnight (the longer you leave them, the more rum they'll soak up).
Line an 8x8-inch tin with baking paper.
Break the chocolate into pieces and melt in a saucepan over low heat on the stove. Once it's fully melted, remove it from the heat and stir in the condensed milk.
Drain the raisins to get rid of any leftover rum then mix them into the fudge mixture, setting a few aside to top if desired.
Transfer the mixture to your lined tin and press it down firmly into place. A rubber spatula is a great help for this part. Top with the remaining rum-soaked raisins.
Refrigerate the fudge for 2-3 hours until firm. Then remove it from the tin, cut it into squares and enjoy!