How do you spell damn it?

I've tried both dammit and damnit, but they both show up as wrong in my spellchecker. I'm trying to use it in the expletive fashion. Like, "Dammit/Damnit, I've lost my car keys!" Which one is technically correct? Damnit or dammit? Or can I say, "Damn it, I've lost my car keys!" Would that be the same as damnit/dammit? The spellchecker for Yahoo is underlining damnit as I type this, but not dammit. So is dammit the correct spelling?