Marshmallow

Eaten straight from the stack it gives the shortest Well Fed window in the game, short enough to run out before you have finished walking to the cave you were heading for. Put it on a stick and hold it over a fire and the same marshmallow feeds you for an order of magnitude longer, at the cost of one piece of any wood. There is no situation where swallowing one raw is the right call.

The Merchant only lists them while he is standing in a Snow biome, so either move him into a snow house permanently or walk over and trade there. If you plan to keep Cooked Marshmallows on hand as your standing food buff, buy in bulk on the trip rather than making the walk twice.

The tooltip asking how many fit in your mouth is a nod to chubby bunny, the party game where people cram in marshmallows until they cannot say the words.