They sell it at the harry potter studio tour in London. Its vanilla soda topped with sweetened clotted cream nothing else.
Its pretty gross.
Completely off topic:
If you like harry potter and are ever in London England the studio tour is an absolute must. Its amazing everything has so much detail. We will be going back next trip. I'll skip the butter beer next time though....
Next: Irish Red Ale, Mild, BC Pale, Scotch Ale, Helles, Pils
Planning: Bombardier clone (The real ale cask version, not that sh!te in the bottles)
P1: Chocolate Oatmeal Stout P2: Pear Wine P3: Air P4: Oxyclean
Keg 1: Stout Stout Keg 2: Dead Intruder Mailbock