Peanut flour is made by grinding the roasted nuts into a thick paste, and then pressing out most or all of the oil. The dry rubble left behind is then further ground into a fine powder that can be used in cooking. Flour made from lightly roasted peanuts will only have a slight peanut flavor, whereas the flour made from dark-roasted peanuts will give food a more robust and forward peanut flavor.
The list for how to use peanut flour stretches out the door. You can add a few tablespoons to smoothies for a protein boost and to add thickness. Likewise, mix it with soups and sauces to thicken and give a creamier texture. Peanut flour can be used for breading meat and fish, made into a creamy vinaigrette, or even reconstituted with water for a low-fat peanut butter substitute.
When used in cookies, muffins, cakes, and breads, peanut flour adds a subtle nutty essence. It won't form gluten like wheat flours, so it's best when used in combination with wheat flours or gluten-free flour mixes.
Some gourmet food stores and co-ops will carry peanut flour, but you'll have more luck finding it online (see below). Since the most of the oils have been removed, the flour will keep in the cupboard for several months without going rancid. If it's very warm, or if you use the flour very infrequently, storing the flour in the fridge or freezer can extend its shelf-life.
Do you have peanut flour in your cupboard? How do you use it?
Sources for Peanut Flour