If I put peanuts in a coffee grinder, will I get peanut butter or peanut powder?
While the grinding of peanuts produces a paste we call peanut butter, a food processor may be slightly better equipped to handle them then a coffee grinder, especially if you want to blend coffee with it again. 
Peppercorns are harder and they can be done in a coffee grinder, but peanuts would likely result in a mess that would not easily be cleaned, and somewhat chunky peanut butter.

edit:

As stated in the comments below you may get some good headway with a blender. I had a crappy blender at home, but peanuts may be easier to grind than thick smoothies. But again, cleaning may take some time afterwords. But blender would probably be more effective than a coffee grinder. 

Peanut powder can be made by freezing the peanuts I believe, very cold. A part of the modern cuisine. I think you would need liquid nitrogen for it. Source from Tech stuff podcast, title of the podcast was Molecularly Gastronomical.