I get Seimei to summon a shikigami and steal the cookie again from you

I sneak in while you're sleeping and replace the cookie with an identical plastic replica, thus stealing the cookie and, again, running away. xD

I called a cop to deal with you, then take cookie back myself

The cop is actually my friend in disguise. He tells you he has to confiscate the cookie as evidence, and then brings it to me.

Turns out he take the plastic replica instead, so the cookie is still with me

I send a kitten into the house, and while you're distracted with the cuteness, I sneak in and steal the cookie.

I get Hikaru to create a magic seal to bind you, then i take the cookie back from you

I send Hotori in, and while she's rambling at you and keeping you distracted, I steal the cookie.

I get Doctor Who to distract you and get the cookie back myself from you

That would work really well, actually. I don't think I'd care about the cookie anymore. xD

Anyway, I disguise myself as a Dalek, scare you off, and then steal the cookie.

I summon a real Dalek to deal with you, and directly steal the cookie back.

Since I'm dressed as a Dalek, the Dalek thinks I'm on it's side, so I tell it we have to have to cookie for the good of the Dalek race. It helps me steal the cookie back and then goes off to exterminate stuff.

I'll terminate the Dalek myself, and steal the cookie directly from you

I hire Moriarty (from Sherlock) to mastermind an elaborate plot to steal the cookie back from you, and replace it with an identical cookie so you can't tell the difference. I then hide out in London with the cookie.

I went to London myself and hire Holmes to become my aid.
Then, i'll steal the real cookie back from you