Now, let me preface this by saying that I do not like carbonated drinks (well, my body doesn't) so what I did was boil the cream soda until the carbonation was gone.
In a separate pot, take 3 cups of the cream soda, mix it with the two cups of milk, add the 1/4 cup brown sugar, 2 tablespoons of butter, and your 2 tbsp of butterscotch and mix together. You do not necessarily want this to boil, so, keep it on a medium heat and get it to the point where it is steaming.
While this is happening (should you choose to drink it warm) begin to make your whipped cream topping. It is really how much you want to make / use so, I did not put an exact measurement.
If you are serving it warm, remove it from the stove, put your cream on top, and drink up!
If you want to try it cold, transfer your mixture to a pitcher and place in the fridge for at least a few hours.
When you decide to serve it, you can serve it over ice, or, not, that is your call. But you should make some whipped cream to put on top nonetheless.
Drink up and experience the bliss that is butterbeer!