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 two 10- to 12-ounce mugs. Fill mugs with heated cream soda and stir thoroughly. Serve garnished with an old-fashioned butterscotch candy stick.