In a medium saucepan (preferably not non-stick) heat the sugar over medium heat. Stir constantly with a rubber spatula as the sugar melts. This may take a while, and it will start with the sugar clumping together, but just keep stirring and it will eventually all melt; if you don't stir some of the sugar can burn in the pan.
Once the sugar has entirely melted, it should be a nice dark brown color. Immediately stir in the butter, I recommend cutting it into 5-6 pieces to help it melt faster. Once the butter has entirely melted and mixed with the sugar, let it cook for 1 minute without stirring. It will bubble and foam while this is happening, and that's normal.
Slowly stir in the heavy cream. Once the cream is added and fully mixed together, again let it boil for 1 minute without stirring. Once done, remove from the heat and stir in the salt and vanilla extract. Stir well to make sure the salt dissolves.
Let the sauce cool a bit and it will be ready to use. It stores well, up to 1 month if tightly sealed in the fridge. It will thicken as it cools down, and becomes quite hard in the fridge so ~20 in the microwave will be necessary to make it back into a proper sauce.
Enjoy!
