1½ cups cream soda
7 tablespoons butterscotch schnapps (or butterscotch syrup like Torani), divided
2 cups ice
6 tablespoons heavy cream
Combine cream soda, 6 tablespoons schnapps and ice in a blender. Blend until slushy. Pour into a glass or mug.
In a small bowl, combine heavy cream and the remaining 1 tablespoon of schnapps. Using an electric mixer, whisk together until almost whipped. You want the cream mixture to be slightly loose, not as firm as whipped cream.
Top the butterbeer with the foamy topping and serve immediately.