THE INTERNET HAS A BUTTERBEER RECIPE.

from here.

ingredients:
1 cup (8 oz) club soda or cream soda
½ cup (4 oz) butterscotch syrup (ice cream topping)
½ tablespoon butter

directions:
step 1: measure butterscotch and butter into a 2 cup (16 oz) glass. microwave on high for 1 to 1½ minutes, or until syrup is bubbly and butter is completely incorporated.
step 2: stir and cool for 30 seconds, then slowly mix in club soda. mixture will fizz quite a bit.
step 3: serve in two coffee mugs or small glasses

and some tips from my experience making it

• it says cream soda or club soda so I made it with club soda because that it what we found in the market and I recommend doing that, I think it would be too over-the-top sweet with cream soda
• don’t put the club soda in the fridge before, let it reach room temperature because you want the drink to stay hot
• it tastes best if you are sitting on a stool in your favorite inn/pub, wearing your gryffindor scarf, and chilling with your best friends harry, ron and hermione (I would know)