The definition of cannot should be either "the negative form of can" (as the AHD has it) or a periphrasis like "is not able to." The only context in which can not, two words, occurs is as an emphatic alternative: "You can do it, or you can not do it." In that case, it is clearly two separately spoken words, with the not given special emphasis, and equally clearly it means something very different from cannot, namely "have the option of not (doing something)." The only acceptable form for the unabbreviated negative of can (or, if you prefer, for the expansion of can't) is cannot, one word.