How does who pronounce them? Depends on where & which accent. . As others say, cookie has a shorter vowel sound, but in my accent, also a different sound.

the oo sound is shorter in cookie!

In standard British English with contemporary received pronunciation or a similar accent, the first syllable in cookie is the same as the word cook, and the word cookie rhymes with bookie, meaning a person whose business involves accepting bets (gambling), usually related to sport. In standard British English with contemporary received pronunciation or a similar accent, the first syllable in kooky rhymes with the word mook, juke or fluke. I think that means it's a diphthong, but I'm not an expert on that sort of thing :-)