Okay, so, this is connected to this post. It will make little to no sense without you having seen that one first. So go on... have a look and then come back here if you want.
You'll want to use something like one of these tools to scoop out the peanut butter batter. I prefer the cookie scoop, but if you don't have one a table spoon will do just fine.
Start out with about six or so little balls of batter on top of the brownie batter. You can add more later.
Spread the out a bit so they look like little splashes of batter, as though somebody threw them at the pan. I wouldn't suggest doing that, but if someone wants to try it, lemme know how it goes.
Keep spreading the batter until it looks something like this. Make sure it reaches to the edges, but you can leave a blank space here and there if you really want.