To make hot Butterbeer: In a small saucepan over medium heat, combine beer and 3 to 4 tablespoons butterscotch sauce. Whisk occasionally, and cook until beer just begins to boil. Taste, and adjust with more butterscotch if necessary. Transfer to a mug.
To make cold Butterbeer: Pour cold beer into a blender. As blender is running, carefully pour butterscotch in a steady stream. Blend until just combined. Avoid overblending (which can flatten the beer). Taste, and blend with more butterscotch if necessary. Transfer to a mug.