Harry Potter Butterbeer Recipe. For butterbeer: 2 liter cream soda, 2 T butter extract, 2 t rum extract. Drop the extracts into the 2 liter bottle, put cap on, slowly rotate until combined. For Cream Topping: One 7 oz container marshmallow creme, 1 cup whipping cream, 1 t rum extract. Add all items together in an electric mixer, combine until smooth. Lastly: pour butter beer into a glass, drizzle with cream topping, enjoy!