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The title refers to the cat named Zazzles (whom Sheldon thinks is Zazzy) that he gets to replace Amy in his life after they break up.
Jim Parsons has revealed in an interview that he, much like Sheldon, is allergic to cats.
In The Fuzzy Boots Corollary (2007) it is implied that Sheldon can't have a cat because he has asthma.
When Sheldon fills the apartment with cats, he obviously forgot that he mentioned in The Plimpton Stimulation (2010) that pets are banned per the Roommate Agreement, with the exception of service animals, such as seeing eye dogs and cybernetically enhanced helper monkeys. It's possible, though, that Sheldon is considering the cats to be emotional support animals, which many people erroneously consider to be service animals.
What Americans call "Danish pastry" the Danes call "wienerbrod" (Vienna bread). The original style of layered pastry consisting of: dough, butter, dough, butter, dough goes back to 14th century Vienna. The Danes modified the recipe, making today's Danish pastry similar to the croissant, a French modification of the wienerbrod recipe.