Place brown sugar, water and sweetened condensed milk in a saucepan over medium-low heat. Whisk together until a smooth mixture remains.
Bring mixture to a simmer and simmer for 15 minutes, whisking frequently. Clumps will form if you do not whisk vigorously enough while simmering.
After 15 minutes, remove from heat and mix in vanilla extract and salt.
Allow the syrup to cool in the sauce pan whisking occasionally as it cools taking time to whisk out any clumps
Transfer to a glass jar and store in the fridge for up to one month.
Notes
Keep it at a bare simmer. Sweetened condensed milk will split or turn grainy if it comes to a hard boil. Low heat, whisk every few minutes, and don’t walk away from it for long.Add the vanilla off the heat. Stir it in once the pan is off the burner so the flavor doesn’t cook away.It thickens as it cools, so pull it when it coats the back of a spoon and still pours easily. If you reduce it until it looks like the finished syrup in the pan, it will be too thick cold. A short warm-up with a splash of water loosens it if that happens.To serve: 2 tablespoons per 12-ounce latte, or 3 for a venti-sized cup. Also excellent stirred into iced coffee or cold brew, where it works as both the sweetener and the cream.For the crunchy brulee topping, sprinkle about a teaspoon of toffee bits over the whipped cream or cold foam right before serving. Add them any earlier and they soften into the milk.Storage: keep in a sealed jar in the refrigerator for up to one month. It settles as it sits, so shake or stir before each use. For a dairy-free batch, swap in sweetened condensed coconut milk one for one.