Pour the sugar into a heavy 2 quart (2 lt) saucepan and heat on the stove over a medium flame. Stir the sugar as it heats. After a 1-2 minutes, you will notice that the sugar begins to clump. The sugar will begin to caramelize and melt as you continue to stir. 

When the sugar has melted and is golden brown, add the melted butter and continue to stir. Add the salt and turn the heat down to a simmer. While stirring, pour the heavy cream in a thin stream into the caramelized sugar. Continue to heat for another 30 seconds while stirring. 

Once the butterscotch sauce is smooth and free of lumps, remove from the heat and allow to cool. Serve warm or store in the refrigerator for later use.