I'm trying to make mug cookies and it calls for cookie butter, which I don't have. Can I use peanut butter instead?
Peanut butter and cookie butter are quite different in taste: your end-result will be very different as well.
To make your own cookie butter, take 66% gingerbread cookies, 33% unsalted butter (left out of the fridge) and mix into a paste.
That being said: if you really like peanut butter and you're making this for yourself: go for it: the texture is about the same. I would add a bit of honey or jam to the peanut butter as well as cookie butter is a bit sweeter then peanut butter.