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. Remove from heat; slowly add cream, whisking until combined. (Mixture will bubble up.) Add butter, a few cubes at a time, whisking until combined after each addition. Stir in vanilla and salt. Let cool to room temperature before using.