Put the chestnuts on a dehydrator tray and dry on medium heat (90F) for 3 days or until bone-dry. Drying chestnuts at a higher temperature can cause them to turn black.
Alternately the nuts can be dried on a tray with a fan blowing on them, they may take over a week to dry like this.
Crack the chestnuts and remove any inner brown skin, which can make the flour bitter.
Crush each chestnut with a hammer or in a mortar and pestle to make them easier to grind.
Put the cracked chestnuts in a Vitamix dry bowl or grind with a grain mill.
Grind the flour a second time in a spice grinder, then sift through a fine sieve to remove large particles.
Grind the leftover pieces a second time, sifting and repeating to get the finest flour possible.
Store the flour in a jar in the refrigerator or freeze for long term storage.
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Notes
Note that homemade flour will always be more coarse than commercially made flour. In the recipes on this website I've noted different ways the flour can be used, but the finely ground commercial flour is the easiest to work with.