The correct one is 'can't'. It is what we call a 'contraction'. It is made from combining the two words 'Can' + 'Not'. We put the apostrophe ( ' ) where we dropped the letter (Got rid of 'o' so put an apostrophe there).
'Cann't' is not actually a word. When we write can't we drop one of the 'n' letters, I'm not sure why but that's just how it is.