singular / plural
hamentasch / hamentaschim
hamentaschen / hamentaschen
I believe the second option prevails colloquially. Is there a correct/incorrect usage of this Yiddish word? Does it matter?
I went looking for instances of "hamentash" (המן־טאַש) in Sholom Aleichem stories because I wanted to see it used in a sentence as singular and plural. In the story "Two Shalachmones or A Purim Scandal" the word is used 12 times, but every single usage is singular. It is consistently spelled המן־טאַש or "hamentash". You can find the first instance on page 58 (page 62 of the file) in the paragraph at the bottom of the page. The next eight instances are all on page 60 (page 64 of the file) in a humorous jabbering conversation between the two women characters.
Regarding your question about the Yiddish spelling, a Google search returns results for spelling the plural with a "ן", but not with a "ם" at the end. It looks like the evidence so far confirms Wikipedia's assertation that:
Wikipedia does not give a spelling for the Yiddish plural, but Google searches reveal two common spellings "המן טאשן" and "המן-טאשן" (with a space or with a hyphen).
One story that I know has the plural hamantashen is "Two Dead Men" about Chlavne the drunk and his wife Gittel, but I cannot seem to find that in the original Yiddish.