I mentioned on Facebook that I'd tried this recipe and had to tweak it last week and that it turned out really well. Since several people requested recipe, I thought I'd post it here:
1 box chocolate cake mix (I know you hate it when I make it easy like this). I used German chocolate, but I'm sure any type would do. Alternately, if you only have white or yellow cake mix on hand, you could add half a cup of cocoa powder to it before the other ingredients and that will work just as well. Alternately, if you just want peanut butter without the chocolate, start with a white or yellow cake mix instead.
1 Cup sour cream
3/4 Cup creamy peanut butter (I used the end of my chunky, and then had to deal with bits of peanuts in the cupcakes, which was fine, but creamy would have been better).
That was the original recipe, but it was WAY too thick, so I added:
2 eggs, slightly scrambled
and about 1 cup of milk--sorry, I didn't actually measure it, I just poured until it came out at a consistency that I knew would work. My milk was in a quart jar, though, so I know it was close to a cup. It was still thicker then normal cake batter--just a little thinner than brownies--but turned out great. You can always add a few more tablespoons of milk if it needs more thinning.
For cupcakes: bake at 350 for about 15 minutes. I touched the tops for springy-ness before removing them from the oven.
For a cake, follow the directions on the box of cake mix and it should come out close to that time--just keep an eye on it.
I topped my cupcakes with chocolate frosting because I really love chocolate!