Safire 1984 quotes a correspondent who uses the spelling dassent and another who wonders if it shouldn't have been dassn't. This contractoin - from dares not, apparently - was common in the 19th century and the early 20th (theDictionary of Amerian Regional English shows many spellings) and was used for dares not, dare not and dared not. The spelling variations are presumably intended to approximate speech. Dassent as a spelling is neither the most frequent nor the rarest:
... I whipped Ed Walker twice, Saturday. I don't like girls. You dassen't catch toads unless with a string..." - O. Henry, "The Ransom of Red Chief," 1907
Dassn't (now the commonest form) and its variations are basically dialectal but, as te use by a correspondent of Safire's suggests, are among those countrified terms trotted out for effect in otherwise straightforward writing.
Like those beetles on the waterpond, you can bend the surface tension film but you dassn't break through - Christopher Morley, The Man Who Made Friends With Himself, 1949
... chortling openly at the things a bigot thinks but dassn't utter - Jack O'Brien, Springfield (Mass.) Union, 1 Dec. 1973
I like dassn't.