If there are any golden legendaries that you specifically want, it's best to craft them before opening packs due to the No Duplicate Legendary rule for opening packs.
Say you wanted a Golden Sir Finley, by crafting him, you remove that card from the pool of legendaries you can get from packs.
According to the card back info in-game, the Cupcake card back can be acquired "by purchasing it during the Scholomance Academy launch period."
I imagine it will start on Aug. 6th and be available for a few weeks.
If there are any golden legendaries that you specifically want, it's best to craft them before opening packs due to the No Duplicate Legendary rule for opening packs.
Say you wanted a Golden Sir Finley, by crafting him, you remove that card from the pool of legendaries you can get from packs.