Blizzard seems to be bringing back oldschool card backs to the shop and make them available for new players. A while ago, Naxxramas card back made it back to the shop. And now another one is available (thus we assume that more “popular” card backs will be back in the coming months).
Originally, Cupcake card back was awarded for achieving Rank 20 during Ranked Season 13 (April 2015).
The card back is available for either 500 Gold or $2.99 / €2.99. It’s a limited time offer, but we don’t know how long will it last – but the Naxx one was available for a few weeks, so you probably don’t need to hurry up if you want to get it.