I have a love of Victorian and early 20th century advertisements.
So, I shuffled the 'Butterbeer Vintage' design elements to make this poster look more like an old-fashioned ad.
I can totally imagine this poster hanging in the window of a wizarding shop somewhere--right next to a poster for Bertie Botts and Cauldron Cakes.
Poster prints are available here: [link]
The original t-shirt is here: [link]