In a medium-size sauce pan, melt the butter over medium heat. Add the brown sugar and lemon juice and stir to combine. The mixture will look like wet sand.
Stand back a little (as the mixture will sputter a bit) and add the heavy cream. Stir to combine then return to a low steady boil. Cook for another 4 minutes, stirring frequently. Add the vanilla, Scotch (if using) and sea salt. Sir to combine.
See Café Tips above in the post for more detailed instructions and tips.
Makes 2 cups of butterscotch sauce.