The Gemara (Brachos 37a) writes that the Bracha on raw “orez” is Hoadama, and on cooked “orez” which partially disintegrates or sticks together is Mezonos.
There is a dispute in the poskim as to whether orez refers to rice or to millet. The accepted opinion is that orez refers to rice (see Mishnah Berurah 208:25).
However, raw or toasted rice kernels are BoreiPriHoadama (Shulchan Aruch Orach Chaim 208:4,7). Rav Moshe Feinstein zt”l (Igros Moshe Orach Chaim 4:45) writes that puffed kernels are Hoadama as well. Because rice cakes are made from puffed rice, based on the aforementioned, some recite a Borei Pri Hoadama.
However, Rav Belsky zt”l said in the name of Rav Moshe Feinstein zt”l that the correct Bracha for rice cakes is Mezonos. Although individual puffed rice is Hoadama, in this case, the rice fuses together and forms a cake. Therefore, it is like cooked rice that sticks together whose Bracha is Mezonos.
Blessed are You, Hashem our God, King of the universe, Who creates numerous living things with their deficiencies; for all that You have created with which to maintain the life of every being. Blessed is He, the life of the worlds.
Baruch Atah Adonai Eloheinu Melech Haolam, Borei Nefashos Rabos V'chesronan Al Kol Ma Shebarasa L'hachayos Bahem Nefesh Kol Chai, Baruch Chei Haolamim.