In a heavy-bottomed saucepan, combine the sugar and water over medium-low heat until the sugar dissolves. Increase the heat and bring to a boil, without stirring. If necessary, use a wet pastry brush to wash down any crystals on the side of the pan. Boil until the syrup is a deep amber color, about 5 to 6 minutes.
Remove the sugar from the heat and carefully whisk in the heavy cream, adding only a quarter cup at a time and thoroughly whisking before adding the next. The mixture will bubble and steam. Stir in the butter and salt. Allow to cool. Store in a refrigerator for up to 2 weeks.