Make the butterscotch: melt the butter in a heavy-bottomed saucepan. Add the dark brown sugar and the salt can cook until the mixture comes to a boil. Stirring constantly, cook until the mixture breaks apart and then comes together again and just begins to smoke.
Immediately pour in the heavy cream. It will bubble and spit, so be careful, and the mixture may harden up. Just keep stirring over medium-low heat until all the butterscotch melts back into the cream.
