Place butter, cream, salt, and sugar in sauce pan over medium-high heat. Whisk continuously. Once this begins to simmer turn the heat down to medium and set timer for 4 minutes. You want the sauce to “simmer” not turn into a “full rolling boil.” Cook over medium heat for 4 minutes. Do not over cook.
After 4 minutes, remove from heat. Allow to stop bubbling (about 15 seconds) stir in chocolate chips with a wooden spoon. Next, stir in marshmallows and vanilla. Continue stirring until combined. Add nuts if desired. Transfer into foil lined pan & put in refrigerator to cool.