Marshmallow on a Stick

You have to be genuinely holding it, either selected in the hotbar or stuck to the cursor, and standing so the stick overlaps the fire itself. Switching to another item, stepping out of the flame, or the Campfire being switched off all reset the progress completely. Not paused, reset. A wired campfire on a timer is a good way to ruin several attempts before you work out what is happening.

It has to be a campfire. A fireplace looks like it should work and does not. The time it takes is randomized, so two attempts at the same fire can feel very different, and there is no way to speed it up: Journey Mode time controls do nothing here. Start one going while you are sorting chests and check back.

The cooked result lasts long enough to be a standing food buff instead of a novelty, and the input is a Marshmallow and one piece of wood. Cook them in batches when you are already parked at base.