In a small saucepan, combine the sugar, water and rum. Heat to boiling stirring occasionally. Cook until the sugar has dissolved. Pour the syrup over the bowl of raisins and let cool to room temperature.
Place a fine mesh strainer over a glass measuring cup and pour the raisins and syrup through the strainer. Reserve raisins for garnish. Use the raisin simple syrup in the recipe.
FOR RUM RAISIN OLD FASHIONED:
Place one large ice cube in an old fashioned glass. Add the rum, raisin simple surup, allspice dram, orange bitters and angostura bitters. Stir well to combine. Garnish with an orange twist and/or plumped raisins.