I haven't yet tried this recipe, but it keeps cropping up all over the internet as the best butterbeer recipe and the closest match to the one found at The Wizarding World of Harry Potter. Just as a warning, it is a bit complicated, but I'm told it's the best, so it's worth the bit of extra work.
Some people recommend chilling it before serving, others freezing it (so that it makes a kind of slushy mixture) and others say to heat it so it's all bubbly and warm. Whatever your preference, try it out and let me know how it is! I'm planning on making it myself either this weekend or next because the weekend after next I'm having a HUGE Harry Potter marathon party for my birthday :) Squeee!
*UPDATE: I tried this and let me tell you, it was fan-flipping-tastic. Like I absolutely adored it. So yummy, oh goodness.
•1 cup light or dark brown sugar
•2 tablespoons water
•6 tablespoon butter
•1/2 teaspoon salt
•1/2 teaspoon cider vinegar
•3/4 cup heavy cream, divided
•1/2 teaspoon rum extract
•Four 12-ounce bottles cream soda
In a small saucepan over medium heat, combine the brown sugar and water. Bring to a gentle boil and cook, stirring often, until the mixture reads 240 F on a candy thermometer. Stir in the butter, salt, vinegar and 1/4 cup heavy cream. Set aside to cool to room temperature. Once the mixture has cooled, stir in the rum extract.
In a medium bowl, combine 2 tablespoons of the brown sugar mixture and the remaining 1/2 cup of heavy cream. Use an electric mixer to beat until just thickened, but not completely whipped, about 2 to 3 minutes.
To serve, divide the brown sugar mixture between 4 tall glasses (about 1/4 cup for each glass). Add 1/4 cup of cream soda to each glass, then stir to combine. Fill each glass nearly to the top with additional cream soda, then spoon the whipped topping over each.
Enjoy your newly made butterbeer by curling up with your favorite Harry Potter book or watching your favorite Harry Potter movie!