"Marshmallow Surprise" is a short story in More & More & More Tales to Give You Goosebumps.
Plot
Marsha Zane and her brothers Ricky and Ronnie are having a sled race when 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 Cocoa. The children are reluctant and nervous, but eventually enter her house. While they do so, the full moon begins to rise in the sky. Mrs. Spooner makes them cocoa- a family recipe which she calls "Marshmallow Surprise". Oddly enough, the cocoa has no marshmellows 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 lists off the offenses the children have done to her, including breaking her window, and now destroying her mailbox. Uncomfortable, the kids beg Mrs. Spooner to let them leave, but she refuses. She tells the children that soon their bones will turn to mush, that they are the the marshmallow surprise.
Marsha tells Mrs. Spooner she will regret not having let them go. The full moon hangs in the sky, and the children turn into Werewolves. Mrs. Spooner screams in horror, and the children eat her.