Combine dark brown sugar, water, and salt in a small saucepan and stir to combine.
Bring the mixture to a boil.
Once boiling, lower the heat to medium low and let simmer for 10 minutes.
Remove the mixture from the heat, add in the vanilla extract, and stir to combine.
Let the syrup cool completely before transferring to an airtight container, and store in the fridge for up to three weeks.