Can I pat my female co-worker on the butt and tell her what a good job she did?
Do you want to pat your female co-worker on the butt as a form of compliment?
Yeah, I like her and I don't think she'll mind.
Touching your female co-worker in the manner you describe is wrong. Touching her like that is a form of sexual harassment and it's not appropriate. You will probably be reprimanded for you behavior if you do it.
So you are telling me that I can't approach her with my feelings? Since when is showing appreciation considered a crime?
It is great that you want to show her appreciation for her good work, but is patting her on the butt the best way to do that? If she is offended by your actions, you could lose your job. Maybe you could suggest she get a bonus instead.