"Marshmallow Surprise" is a short story in More & More & More Tales to Give You Goosebumps.
Marsha is racing her sled against her brothers Ricky and Ronnie, when they take a sharp curve into Mrs. Spooner's yard. Mrs. Spooner is a bitter old woman who hates kids. She once called the cops because Marsha's dog was in her yard. In the past, the kids have retaliated by playing tricks on her, such as ringing the bell and running away, but now they've accidentally destroyed her flower garden. Marsha swoops into the yard and knocks over the mailbox with her head. Mrs. Spooner comes out into the yard and instead of getting mad at the kids, she invites them in for cocoa.
Mrs. Spooner serves them the hot chocolate in huge cups and when Marsha asks for some marshmallows, Mrs. Spooner mysteriously tells her that they're coming. The kids finish their cocoa and one of them casually mentions that they'd like the recipe. Mrs. Spooner reveals that the recipe is a secret. Then she starts listing off the horrible things the three kids have done to her and her house. The kids urge Mrs. Spooner to let them leave, but Mrs. Spooner has a surprise for the kids. The marshmallows they were promised? The secret part of the recipe is that it turns whoever drinks the cocoa into marshmallows. Mrs. Spooner tells the kids that they can expect their bones to melt.
Marsha tells Mrs. Spooner that she really should have let her and her brothers go. The three kids start baying at the moon and open their mouths to reveal fangs. The old woman cries out, "You are werewolves!" and they devour her alive.