Add the dark brown sugar and water to a small saucepan and cook over medium-high heat until the sugar dissolves and it begins to simmer, about 10 to 15 minutes. Stir occasionally.
Remove from heat and stir in the vanilla extract.
Let the syrup cool to room temperature in the pan for about 1 hour.
Bottle into an airtight container and store in the refrigerator for up to 1 month.
