Xy'rath's Booby-Trapped Cache
Drops from Xy'rath the Covetous. After lots of times using this until it vanished I'm pretty certain you can only get the cake or the outfit from the cache and never both at the same time.
Xy'rath's Booby-Traced Cache is a toy you can purchase from Xy'rath the Covetous in Zereth Mortis that spawns a lootable chest at your target location.
Guide awaiting review
Purchase toy from vendor
Find Xy'rath the Covetous at 63.4, 49.2 in Zereth Mortis and purchase Xy'rath's Booby-Trapped Cache from them.
Use toy and loot chest
Target a location and use the toy to spawn the cache. Attempt to loot it—you may encounter smoke bombs or webbing effects, but you can retry until you receive loot.
Receive your reward
The cache grants one of two items: either 
Xy'rath's Spare Outfit or 
Red Velvet Delicacy, but never both from the same use.
Tips
You can only receive one item type per cache opening—if you obtain the outfit, you will never receive the cake from future uses, and vice versa.
If you want a specific reward from future cache uses, keep the unwanted item in your inventory to force the cache to grant the other reward instead.
The physical cache can be stood upon for slight height gain, which may be useful in certain situations.
The cache is interactable for other players, making it shareable in groups.
