In a saucepan over medium-high heat, add brown sugar and cold water and bring to a boil.
Reduce the heat to low and stir constantly until the sugar completely dissolves.
Let the simple syrup simmer for 5-10 minutes. The longer the syrup simmers, the thicker it will become once cooled.
Remove the simple syrup from the heat, stir in vanilla extract (optional), and let it cool.
Use it as desired, and store any remaining simple syrup in an airtight jar in the fridge for up to 2 weeks