Hi, #2 does not seem to me like a natural thing to say. The whole point of this structure is emphasis, so the natural thing is to say the whole thing without abbreviation in order to stress the word 'not'. A rough Google search finds 11, 500 for "how could you not" 185 for "how couldn't you" Clive
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.