Combine the brown sugar and water in a small saucepan.
Bring to a simmer over medium heat, stirring until the sugar dissolves.
Simmer for 3 minutes.
Cool, then transfer to a clean jar or bottle. Refrigerate.
Notes
Store brown sugar simple syrup in a clean, airtight container in the refrigerator for up to 2 weeks. Discard it if you notice mold, an unusual smell, or other signs of spoilage.