Rinse the cranberries under cold water and sort through to remove damaged ones. In a medium saucepan, combine berries with water and bring to a boil over medium heat. Stir occasionally for 5-7 minutes.

Once boiling, reduce heat to a simmer, stir in brown sugar, and let dissolve completely. Add a pinch of cinnamon and nutmeg. Simmer for about 10 minutes until sauce thickens and cranberries break down.

Stir in unsalted butter until melted, then add dark rum, mixing thoroughly. Simmer for an additional 5 minutes, stirring frequently.

Remove from heat, let cool to room temperature, then transfer to an airtight container and refrigerate for at least an hour or overnight.