In a medium-large saucepan, bring the soda, salt, milk, and cream to a boil over medium heat. Let it boil for 7 minutes, mixing once in a while to be sure nothing is burning and the mixture doesn't boil over the pan. Turn the heat to low, add the butterscotch chips and whisk until melted and smooth. Bring the heat back up to medium and let it boil for 3 more minutes.