I use the slash because, while there is a clear difference between toffee and butterscotch candies there is little if any difference between toffee and butterscotch sauce. Butterscotch is generally a bit lighter in color I suppose. To produce that effect all you need to do is use light brown sugar instead of dark brown. Otherwise the procedure is the same. You’ll need:
7 ounces (scant cup) dark brown sugar
2 ounces (4 tablespoons) butter
3 ounces (generous 1/3 cup) heavy cream
Combine all the ingredients in a small saucepan and bring it to a simmer.
Simmer it gently until the sugar is completely dissolved. Allow the to cool and thicken somewhat before using. It will also hold almost indefinitely and can be refrigerated for several weeks. For a smoother sauce that will flow better at lower temperatures, double the cream (at least).