It's very similar to the one served at the Wizarding World of Harry Potter theme park, but even richer and sweeter.
1 cup (8 oz) light or brown sugar
2 TBSP water
6 TBSP butter
1/2 tsp Salt
1/2 tsp Cider Vinegar
3/4 cup Heavy Cream, divided
1/2 tsp Rum Extract
4 12oz bottles of cream soda
Step 1:In a small saucepan over medium, combine the brown sugar and water. Bring to a gentle boil and cook, stirring often, until the mixture reads 240 F on a candy thermometer (in case you don't have a candy thermometer, once it's boiling, wait about 55-60 seconds).
Step 2:Stir in the butter, salt, vinegar, and 1/4 cup heavy cream.
Step 3:Set aside to cool to room temperature. Once the mixture has cooled, stir in the rum extract.
Step 4:In a medium bowl, combine 2 tablespoons of the brown sugar mixture and the remaining 1/2 cup of heavy cream. Use an electric mixer to beat until just thickened, but not completely whipped, about 2 to 3 minutes.
To Serve:Divide the brown sugar mixture among four tall glasses (about 1/4 cup for each glass). Add 1/4 cup of cream soda to each glass, then stir to combine. Fill each glass near to top with additional cream soda, then add the whipped topping.