Buckwheat flour is ground from the small seeds of the buckwheat plant. It is not a grain but a relative of rhubarb, and so it is gluten- and wheat-free. It can be subbed for white flour in baked goods.
Substitute 7/8 cup of buckwheat flour for a cup of white flour. For best results, substitute buckwheat flour for no more than about one-quarter the white flour in leavened breads, and no more than half in muffins other baked goods.
Buckwheat flour has a strong nutty taste that can overpower the other flavors in a recipe. For this reason, buckwheat is usually mixed with other types of flour, such as rice or oat, to produce better results.
Gluten-free flours like buckwheat don't rise as well as wheat flours do. You may find your finished baked goods are denser and heavier than you expect. They may also be more crumbly.