Mix ⅓ cup water, brown sugar and sugar in a medium heavy-bottomed saucepan. Cook over low heat without stirring until the sugar dissolves. Turn heat up to medium and bring to a boil for about 5 minutes uncovered or until the mixture reaches about 350F on a candy thermometer. It should darken slightly but be careful not to burn it! Turn off heat.
Slowly pour in cream and add salt. The mixture will bubble violently and it may solidify. Put the pot back over low heat, stirring constantly, until the caramel dissolves and the sauce is smooth, about two minutes.
Stir in vanilla and cool.Sauce will thicken as it cools.Cool to room temperature and pour into pretty jars. Store refrigerated.
Recipe by Jellibean Journals at https://jellibeanjournals.com/caramel-coffee-syrup/