Combine condensed milk, butterscotch topping, and butter in a glass heatproof measuring cup.
Heat in microwave for 1 minute.
Remove and stir until butter has melted and incorporated into mixture.
Meanwhile heat cream soda in another heatproof measuring cup for 1 minute 30 seconds.
Divide butterscotch mixture between 2 (10 to 12-ounce) mugs.
Fill mugs with heated cream soda and stir thoroughly.
Serve garnished with an old-fashioned butterscotch candy stick.