The Bracha Rishona on Kasha - Buckwheat is Borei Pri Hoadama, and the Bracha Acharona is Borei Nefashos.
Buckwheat is not a kind of wheat, nor is it even one of the Chameshes Minei Dagan (five species of grain), rather it is a legume. Therefore, neither a Mezonos nor Hamotzi can be said over a product of buckwheat unless one of the Chameshes Minei Dagan, such as wheat flour, even only in part, has been added to it. Thus, if you eat buckwheat alone, only Hoadama is said (see Igros Moshe, Orach Chaim 65).
לעילוי נשמת
ר' פנחס אליהו בן יעקב יוסף זצ"ל
ר' חיים יצחק בן דוד זצ"ל
In Memory Of
Leon Manaster
Chaim Melcer