http://www.dailywritingtips.com/cannot-or-can-not/
Although my personal Error Alarm buzzes whenever I see cannot written as two
words, both forms are acceptable usage.
Merriam-Webster lists cannot as one word. If you try looking up “can not”
in the online unabridged, you will be sent to a list of suggestions headed
by cannot.
According to the entry in the OED, cannot is the ordinary modern way of writing
can not