Heat the sugar in a heavy saucepan over medium heat while stirring. The sugar will form clumps as it melts before completely melting and turning golden/amber in color.
When fully melted remove from heat and quickly whisk in the cubed butter (the mixture will bubble up when you add it). Make sure the butter is fully incorporated before slowly adding the cream in a small stream. Pouring in the cream too quickly might cause the caramel to seize. Whisk briskly until fully incorporated and smooth.
Finally, stir in the vanilla and salt and pour the sauce into a heat-safe glass measuring cup to finish cooling. Use in recipes that call for caramel sauce or to top ice cream or lattes. Enjoy!