Mix the brown sugar, half-and-half, butter and salt in a saucepan over medium-low heat. Cook while whisking gently for 5 to 7 minutes, until it gets thicker. Add the vanilla and cook another minute to thicken further. Turn off the heat, cool slightly and pour the sauce into a jar. Refrigerate until cold.
Recipe by the modern barista at https://themodernbarista.com/index.php/2016/06/11/caramel-ribbon-crunch-cupcakes/