I know that dictionaries say the word is inflected muffins (singular), muffinsen (definite article), muffinser (plural). However, doing a Google suggests to me that "muffins" is also commonly used as a plural form, as well as "muffinser". In fact NAOB itself, after saying the plural form is "muffinser", gives the following examples:
(I also wonder what those muffins were in the Norway of 1958. In Britain, anything called a muffin back then would not be a cake!)
- baker Stinton … trakk en plate nybakte muffins ut av ovnen i sitt bakeri (VG 04.02.1958/7)
- hver fredag er det bakedag, da lager ungene hjemmebakte muffins (Aftenposten 12.12.1987/27)
(I also wonder what those muffins were in the Norway of 1958. In Britain, anything called a muffin back then would not be a cake!)