Place sugar in a large saucepan over medium heat. Once the sugar starts to dissolve, stir until it dissolves completely, 10-15 minutes.
Once sugar has dissolved, stop stirring and cook over medium heat until the sugar turns a deep amber color, about 1-3 minutes. Don’t walk away during this step! It can burn very quickly if you aren’t paying attention.
Immediately add the cubed butter and whisk constantly until the butter is melted. It will bubble up a lot.
Remove the pan from the heat and slowly pour in the heavy cream while whisking constantly. Be careful as it will bubble up again. Continue whisking until the caramel is smooth.
If clumps of sugar remain, place the pan back over medium heat for a couple minutes to melt completely.
Add the salt and whisk until combined.