Start to finish: 1 hour (10 minutes active)
1 cup light or dark brown sugar
2 tablespoons water
6 tablespoon butter
1/2 teaspoon salt
1/2 teaspoon cider vinegar
1/2 teaspoon rum extract
Four 12-ounce bottles cream soda
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.
Stir in the butter, salt, vinegar and 1/4 heavy cream. Set aside to cool to room temperature.
Once the mixture has cooled, stir in the rum extract.
To serve, divide the brown sugar mixture between 4 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 nearly to the top with additional cream soda, then spoon the whipped topping over each.