Swirling PB into my brownies?

I wanted to add some Reese's Pieces to my brownie mix (regular box brownie mix) but I ate them all! So someone suggested I mix peanut butter into the brownie batter. I was going to use the swirling method-- swirl the PB around the batter with a knife right before the pan goes into the oven, but I'm a bit... show more I wanted to add some Reese's Pieces to my brownie mix (regular box brownie mix) but I ate them all! So someone suggested I mix peanut butter into the brownie batter. I was going to use the swirling method-- swirl the PB around the batter with a knife right before the pan goes into the oven, but I'm a bit skeptical about using PB straight from the jar. Should I water it down first with maybe milk or something?
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