Instructions

1. Grind the oat groats to flour before anything else, and grind them fine. Half-ground groats stay gritty however long the dough rests, because there is no baking heat here to soften them.
2. Melt the coconut oil gently and let it cool to just above setting point before it goes in. Poured in hot it splits the banana; stirred in solid it stays in pieces.
3. Blend the banana, oil, cinnamon and nutmeg first, then work the oat flour in last. Adding the flour at the end lets you judge the dough, and the right dough holds a thumbprint without cracking at the edges.
4. If it crumbles, add water a spoonful at a time. It wants to be stiff enough to scoop and wet enough to be pushed back into shape after it falls.
5. Stir the raisins through by hand at the very end. A blender chops them to paste, and the whole point of a raisin in a cookie is finding it whole.
6. Drop tablespoons onto a lined tray and flatten each into an even round with wet fingers. A domed cookie dries from the outside in and keeps a damp core.
7. Dehydrate at 46 °C for 15 hours. Turn the cookies over once the tops are firm enough to lift without stretching, so the undersides get the same air as the tops.
8. Take them off early for a chewy cookie and run the full time for a crisp one. The difference is in the last few hours, and a cookie can always go back on.
9. Cool them on a rack and keep them in a tin. A chewy one still holds moisture and belongs in the fridge if it is to last more than four days.