In a small saucepan, combine the dark brown sugar, water, and flaky sea salt. Stir the mixture thoroughly to combine all ingredients.

Place the saucepan over medium heat and bring the mixture to a rolling boil, stirring occasionally to help dissolve the sugar.

Once the mixture reaches a boil, reduce the heat to medium-low and allow it to simmer gently for 10 minutes, stirring occasionally.

Remove the saucepan from heat and immediately stir in the vanilla extract until well combined.

Allow the syrup to cool completely to room temperature before transferring to an airtight glass jar or bottle. Store in the refrigerator for up to 3 weeks.