Pour milk, ½ cup cream, sugar, and butter into a saucepan and whisk together. Place over medium-low heat and simmer until sugar dissolves.
Remove from heat and stir in the butterscotch sauce.
Whip the remaining cream to soft peaks and fold it into the slightly cooled butterscotch mixture until smooth.
Divide the mixture into 2 large mugs and top with cream soda. Serve immediately.