In a medium saucepan, combine the water, brown sugar, granulated sugar, butter, and salt.
Bring to a gentle simmer over medium heat, stirring until the sugars dissolve and the butter is melted.
Simmer for 5–8 minutes, stirring occasionally, until the syrup is smooth and lightly thickened.
Remove from the heat and stir in the vanilla extract and butter extract, if using.
Let cool for 10–15 minutes.
Pour into a clean pint mason jar, seal, and refrigerate.
Storage
Refrigerate for up to 2 weeks.
Shake well before each use, as the butter may naturally separate during storage.