In a small saucepan, heat cream and salt over very low heat. Simmer until salt is dissolved. Remove from heat, and set aside.

In a medium saucepan, heat sugar and 3 tablespoons (45 grams) water over high heat, being careful not to splash sides of pan. (It should be the consistency of wet sand.) Use remaining 1 tablespoon (15 grams) water to brush down sides of pan, and stir to help sugar dissolve. (Do not stir once it starts to boil.) Cook until desired light amber color is reached and a candy thermometer registers 330°F (166°C). Remove from heat; slowly add warm cream mixture, whisking to combine. Add butter, a few pieces at a time, whisking until combined. Stir in vanilla. Let cool completely.