Muffin-balls are a type of edible ground berry. They are the size of a tennis ball with a "silky surface" and have small horns on them. The horns are not poisonous, but are not good to eat as they are very sour. The balls smell like fresh bread and when fried in deer fat will almost taste like meat.
Jake Chambers and Oy found some muffin-balls on the road to Calla Bryn Sturgis. The residents of the Calla knew about the patch; there were fresh signs of them in the patch when Jake stumbled across it.