Sift all ingredients into a large bowl then stir with a whisk or spatula until well combined. Store in an airtight container in the refrigerator or freezer. (Will keep, refrigerated, up to 6 weeks, or frozen for up to 3 months.)
You can make your own millet flour by grinding whole dry millet in a coffee or spice grinder until powdered. Bob’s Red Mill sells garfava bean flour. If you can’t find it, you can use chickpea flour instead.