In a small skillet, bring brown sugar and rum to a boil over medium-high heat, stirring until sugar dissolves. Stir in butter until melted. Simmer until sauce becomes thick and syrupy, about 5 minutes. Stir in salt; remove from heat. Let cool slightly before using.
Recipe by Louisiana Cookin' at https://louisianacookin.com/chocolate-po-boy-bread-pudding/