Instructions

1. Steam the sweet potatoes rather than boiling them if you can. Boiled flesh takes on water you then have to remove again, and steaming keeps the sweetness in the vegetable instead of in the pan.
2. Mash it completely smooth while it is hot. Any lump left now becomes a hard pellet that will never rehydrate, and cold mash cannot be smoothed out.
3. Leave the butter, salt and pepper out at this stage. Fat interferes with drying, and seasoning is far easier to judge when you rebuild the mash than when you dry it.
4. Spread a cupful at a time on a lined sheet to 6 mm, working it flat with an offset spatula. Mash is stiff and holds every ridge you leave in it.
5. Run a pizza cutter through it in a grid of roughly 2.5 cm squares before it goes in. Cut squares dry from their edges as well as their faces, which takes hours off the run.
6. Dry at 52 °C for 14 hours, separating the squares and turning them over once the top has gone matt. The underside is against plastic and dries last.
7. They are ready when a square is hard and dry but still bends slightly before it cracks, rather than shattering. Overshooting is not a disaster here, but a brittle square takes noticeably longer to come back.
8. Cool them fully and keep them sealed in the dark for a year. To eat them, cover a handful with the boiling water, put a plate over the bowl and leave it for fifteen minutes.
9. Stir, heat it through, and add the butter, salt and pepper once it is hot. The mash tightens as it comes back up to temperature and the butter is what loosens it again.