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.
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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.
If you're going to roast the peanuts, definitely roast them before grinding. Do like you would any other nuts: in the oven, maybe 350F, spread out not too deep in a pan, until they smell toasty and nutty, making sure not to burn them.
If you ground them into peanut peanut butter, and then tried to roast it, it'd be a lot tougher to roast. You'd have to spread it out in a thin layer on a pan, since if it's thick only the surface would really roast. You'd have to be really careful about overroasting, since you won't be able to directly see if the bottom is scorching. And then you'll have to scrape out the pan to try not to waste any, and still have a greasy pan to clean up. Roasting before, on the other hand, is easy. There's space for air between the peanuts so they'll all roast. You can easily give them a stir if you need to, and see directly whether they're burning. And the pan will basically still be clean when you're done.
Whether or not to remove the skins is mostly down to personal preference. You will end up with flecks of skin in the peanut butter, so if that bothers you, remove them. If you're really sensitive (and eating the peanut butter mostly by itself) it's possibly you'd notice a texture difference too. But they won't taste bad (some people even think they improve the flavor), so do whatever you feel like!
If you do want to remove the skins, you can and should wait til after roasting: they should be much looser and easier to remove after roasting. You might even be able to get a lot off simply by putting them in a bowl or colander and shaking. Otherwise, putting them in a kitchen towel and rubbing around should work pretty well.
Best Answer
About 100 grams of peanut butter will fit into about 6 tablespoons, or slightly under 1/2 cup. If you tasted, and there was peanut butter left on the sides and floor of wet grinder, that could certainly explain the difference. There is really nowhere else for it to go.