To make the caramel sauce, heat sugar in a medium saucepan over medium heat, stirring occasionally with a wooden spoon until the sugar melts into an amber colored liquid. Once the sugar is completely melted, immediately add the butter one piece at a time being careful as the water in the butter will spatter. Stir until butter is all incorporated, 1-2 minutes. Slowly drizzle in the cream. It will boil vigorously and could bubble over so be very careful and stir constantly. Allow the mixture to boil for 1 minute. Remove from heat and stir in espresso vinegar and salt. Allow to cool down slightly before using.