1. In  a small saucepan, combine brown sugar with 2 tablespoons water. Bring  to a boil over medium heat and cook until the mixture reaches 240  degrees, stirring occasionally which is about a minute of cooking. Turn  off the heat and add the butter, salt, vinegar, and 1/4 cup heavy  cream. Stir until smooth. Cool mixture to room temperature before using.
2. In a medium bowl, beat together the remaining cream (1/2 cup) with 2 tablespoons of the brown sugar syrup.
3. In  each glass, mix together 1/4 cup brown sugar syrup with 1/4 cup cream  soda. Pour in the rest of the cream soda until it is nearly to the brim  of the glass. Top with whipped cream.