Combine the sugar and water in a saucepan. Set heat to medium-high and stir mixture frequently. When the sugar has completely melted and the mixture is uniformly bubbly (this will take a couple minutes), add the heavy cream and butter.
Continue stirring for 2-4 minutes, or until butter has melted and the caramel is smooth and has turned a deep caramel color.
Remove from the heat and stir in the salt. Let cool and pour into containers. Cover and refrigerate until ready to use.
Notes
When you pull the sauce out of the fridge you can either bring it to room temperature to use or reheat the sauce. Remove the lid and microwave for about 30 seconds, then stir. It should be smooth and pourable, but not piping hot.
Do not substitute kosher salt or table salt for the sea salt. The result will be way too salty!