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 240F on a candy thermometer.

Stir in the butter, salt, vinegar and 1/2 heavy cream.

Mix in the rum extract.

In a medium bowl, combine 2 tablespoons of the brown sugar mixture and the remaining heavy cream. Use an electric mixer to beat until just thickened, but not completely whipped, about 2 to 3 minutes. (When I made this recipe, I went the fast route and just used a can of whipped cream for this step. Worked out just fine).

To serve, divide the brown sugar mixture between 2 tall glasses (about 1/8 cup for each glass). Add 1/8 cup of cream soda to each glass, then stir to combine. Fill each glass nearly to the top with additional cream soda, then spoon the whipped topping over each.