Add butterscotch sauce to glass. If the sauce has been in the fridge, you will need to add the boiling water as well.
Add the rum (if using), and stir to combine with the butterscotch sauce.
Pour in the soda, and give the mixture a quick stir to combine. If you stir too much, you will lose the carbonation of the drink.
Add the ice cream, and stir briefly until a foam forms on top of the drink.
For a more intense beverage:
Warm the mixture in the microwave for about 30 seconds, then add the ice cream.
Substitutions: If you cannot find Virgil's Cream Soda, try to use another artisan or craft cream soda. You need a brown/caramel coloured cream soda to get the best taste - pink or clear cream sodas do not work as well here. Alcohol warning: Please do not serve alcohol to children, pregnant women and others who should not consume it. If you choose to drink, do not drive.