Brocho before eating this: Take a Crispix and split it. Say Borei Minei Mezonos and eat the rice side [dark side], and then say Borei Pri Ha'adama and eat the corn side [light side].
Brocho after eating the proper shiur of this: Borei Nefashos
Source: Star-K Cereal List
Note: Halachically Speaking (volume 4, issue 2) says that the custom is to say Shehakol Niyeh Bedvoro