If the Muffin is made with grain flour
Brocho before eating this: Borei Minei Mezonos
Brocho after eating the proper shiur of this: Al Hamichya
If one is Koveah Seduah on this item, Hamotzi Lechem Min Ha'aretz and Bentching are required.
If the muffin does not contain grain flour
Brocho before eating this: Shehakol Niyeh Bedvoro
Brocho after eating the proper shiur of this: Borei Nefashos
Source: The Laws of B'rachos (page 365)