Mix 8 oz cream soda with 4 oz light beer and 1 tbsp butterscotch schnapps. In a small saucepan, melt 1tbsp margarine and mix with 2 tbsp brown sugar. Remove from heat and mix 1 cup marshmallow cream and 1 tsp butterscotch schnapps. Pour the cream sugar/beer mixture into a cup and spoon the marshmallow cream mixture over the top.
If you're looking for a non-alcoholic butterbeer recipe, I did share this slushie version last year and it's also pretty delicious. What's your favorite magical beverage?