There are 2 different methods that work. The first is to put the Cream Soda in the freezer for a couple of hours before mixing in the other ingredients. The key is to chill it just until it is slushy. If you freeze the soda, it will lose its fizz. Try tipping the bottle back and forth every 30-45 minutes to help achieve that slushy texture.  The other approach is to mix up the butterbeer and transfer it to a quart-size freezer bag. Remove all of the excess air and nest this bag inside a larger, gallon-size freezer bag. Fill the larger outer bag with ice cubes and add a tablespoon of rock salt, just like if you are making this ice cream in a bag. Wrap the bag in a towel and shake it for 5-10 minutes until slushy. You will have to do it in batches, but it works and can be a lot of fun if you want a slushy version!