I know its been forever since I thought I would try this but yesterday was the day! Found many recipes online but this one seems to be the most "copied" one done by too many to count. I know why. Pretty easy and got good reviews in this house :)
1 c dark brown sugar
2 tbsp water
6 tbsp butter
½ tsp salt
½ tsp cider vinegar
3/4 c heavy cream divided
½ tsp rum extract
4 (12oz) cans of cream soda
In a small saucepan over medium heat, combine brown sugar and water. Bring to a gentle boil and cook, stirring often - until mix reads 240º on a candy thermometer.
Stir in butter, salt, vinegar and 1/4 c heavy cream. Set aside to cool to room temperature. Once mix has cooled stir in rum extract,
In medium bowl, combine 2 tbsp of cooled brown sugar mix and remaining ½ c of heavy cream. Using a electric mixer, beat until barely thickened. You don't want it to be like whipped cream - so about 2 minutes.
To serve, divide the brown sugar mixture between your four glasses (about 1/4c each). Add 1/4 c of cream soda to each glass, then stir to combine. Next add more cream soda, almost to top.  Spoon a dollop of whipped topping over each glass - I used our souvenir cups from Harry Potter World.
I actually made the cream first and just kept it in refrigerator then froze the brown sugar mixture. When I was ready to serve it, I just scraped frozen mix into glass and followed the rest of directions above :)