How to Make It
Combine the sugar and water in a medium, heavy saucepan over medium-low heat; cook, stirring often, until the sugar dissolves.
Increase the heat to medium-high; bring to a boil—without stirring, but instead, regularly swirling the pan and occasional brushing down the sides of the pan with a wet pastry brush. Cook, swirling and brushing regularly, until the syrup turns a deep amber color, about 7-10 minutes.
Gently pour in the heavy cream (the caramel will foam and bubbly up vigorously). Add in the butter pieces and stir gently until smooth. Remove the caramel from heat and stir in the salt.
Use a saucepan larger than what you anticipate needing to contain the listed ingredients, as the caramel will foam up vigorously when the cream is added in step 3, and could bubble over if the pan is too shallow.