Butterscotch syrup: bring the brown sugar, corn syrup and butter to a boil. boil until the syrup reaches 240 degrees (remembering to adjust temperature for altitude)
take off the heat and let cool 5 mins, then stir in the whipping cream, apple cider vinegar and rum extract let cool completely
Butterbeer: stir 2 Tbsp into every 12 oz of cold cream soda - or 3/4 C into a full 2 liters
Topping: beat the marshmallow creme, heavy whipping cream along with 2 Tbsp of the butterscotch syrup together, pour the butterbeer into a glass and add some of the topping. Refrigerate the leftover topping