Pour butter flavor and 2 tsp rum extract into bottle of cream soda, return lid to soda then holding soda horizontally twist and rotate bottle gently to evenly distribute flavorings. Refrigerate until ready to serve.
For the marshmallow cream topping:
In the bowl of an electric stand mixer fitter with the whisk attachment, whip cream on high speed until it begins to thicken slightly and begins to form slight peaks. Add marshmallow cream and remaining 1 tsp rum extract and continue to whip until well blended and smooth. To serve pour soda into mugs and spoon marshmallow cream topping over top.