The secret formula anniversary cake appears in the episode "Plankton Paranoia." The cake was made to celebrate the anniversary of the first day Plankton tried to steal the Krabby Patty secret formula.
The cake has two layers; the bottom is made in the shape of a Krabby Patty and decorated with strawberries and pink icing, and the upper layer is a traditional chocolate cake, also decorated with pink icing. There is a heart with an arrow striking through it on the upper layer's side, and a statue of Mr. Krabs and Plankton fighting over the Krabby Patty secret formula is placed at the very top.
Role in episode
After Mr. Krabs arrives at the Chum Bucket to save Squidward, who is allegedly being held captive by Plankton, he is surprised to find he has been duped into attending a surprise party in honor of the first day Plankton tried to steal the formula. SpongeBob places the cake on top of Plankton. By the time he gets back out of the cake, the party is over and everyone is gone.
Plankton opens a fake secret formula left behind by Mr. Krabs that has been booby-trapped with TNT. The Chum Bucket is completely destroyed by the explosion, alongside the cake.