In a small sauce pan over medium heat combine brown sugar and water. Boil, stir until 240.

Stir in butter, salt, vinegar, 1/4 heavy cream and cool.

Stir in rum extract.

In a medium bowl combine 2 tablespoons of the brown sugar mixture and remaining 1/2 cup of heavy cream. Use an electric mixer to beat until thickened, but not whipped, 2 to 3 minutes.

Divide brown sugar mixture between 4 glasses. 1/4 cup cream soda to each and stir. Add extra cream soda then whipped topping.