During a game of hide-and-seek, the reader finds a fridge in their aunt and uncle's basement. Inside the fridge are two items: a tub of Purple Peanut Butter and a slice of chocolate cake. If the reader chooses to eat the Purple Peanut Butter, they shrink. If they choose to eat the chocolate cake, they grow and become enormous.
After eating the chocolate cake, the reader can seek out Effy in the hopes of getting help. She reveals that the chocolate cake was created by her for a client that was too short, but she added too much "growth spice." Effy agrees to help the reader by making a new cake, one capable of undoing the effects of the chocolate cake from earlier.
In one ending, the reader uses an incorrect ingredient and accidentally turns themselves into a gingerbread person, and Effy decides that she can sell them in her bakery. In another ending, Effy successfully helps the reader, and she gives them some chocolate cupcakes as a gift. The reader accepts the confectioneries, but they decide to throw them away once Effy is out of sight.
Effy tries to help the reader, but she isn't incredibly concerned for their well-being. In one ending, the reader is turned into a gingerbread cookie, and she doesn't seem to care about turning them back into a human.
Effy is described as plump, gray-haired, and having a small head.
- Effy says that she personally knows Kermit Majors, and the reader recognizes Kermit as being a character from the Goosebumps book Monster Blood III.
- Since Kermit lives in Georgia, this may hint that Beware of the Purple Peanut Butter also takes place there.
- Effy's chocolate cake and Monster Blood both cause individuals to grow after consumption.
- Effy's chocolate cake may be an allusion to the cake labeled "EAT ME" from Alice's Adventures in Wonderland. Both cakes make the consumer grow larger.