These are what I call cakey brownies, rather than the dense, chewy sort. There's a place for both, but I really prefer the dense, chewy, chocolate-laden ones.
Having decided on Peanut Butter Brownies, I chose this recipe from Kitchen Travels, after spending quite a long time searching for recipes online. I think it was the gorgeous photos which won me over, as well as the fact that many of the other recipes used chocolate chips in a peanutty cake. I wondered about adding melted chocolate to the chocolate portion of the batter, but decided that anything too chocolatey might overwhelm the peanut butter flavour. The only change I made in the end was to use only white flour, as that's all I had. I needed to add a little milk to the chocolate batter to make it spreadable, but the peanut batter didn't need any extra liquid.
The end result was a good balance of flavours between chocolate and peanut butter, and a well textured moist cake.