"Marshmallow Surprise" is the sixth short story in More & More & More Tales to Give You Goosebumps.
Marsha Zane and her brothers, Ricky and Ronnie, are having a sled race. Marsha accidentally slides into the yard of Mrs. Spooner, completely destroying her garden, and knocking her mailbox over. Mrs. Spooner is a mean, bitter old woman. She once called the police after she caught Marsha's dog digging in her garden, and even called their father after she caught them walking across a corner of her yard. Rumors have even circulated that she buries children in her garden. To their horror, Mrs. Spooner stands before them. Instead of being upset, Mrs. Spooner asks if they are alright and invites them in for hot chocolate.
The children are reluctant and nervous, but they eventually enter the house. The full moon appears overhead as they enter. Mrs. Spooner makes a special batch of hot chocolate using "a family recipe", which she calls "Marshmallow Surprise". Oddly, the cocoa has no marshmallows in it, but the kids enjoy it anyway. After they finish, Mrs. Spooner's demeanor changes, and Marsha realizes she was only pretending to be nice.
Mrs. Spooner begins listing off bad things that the children did to her. Uncomfortable, the kids beg Mrs. Spooner to let them leave, but she refuses. She tells the children that their bones will soon turn into a marshmallow-like mush, hence the name, "Marshmallow Surprise".
Marsha tells Mrs. Spooner she will regret not having let them go. Mrs. Spooner's potion won't work because the kids are werewolves. The full moon hangs in the sky, and the children transform. Mrs. Spooner screams in horror, and the children eat her.