Line an 8" square baking pan with parchment paper, overanging on the sides. Set aside.
In a large mixing bowl, whisk together oil and cocoa powder. Set aside.
In another mixing bowl, mix sugar and eggs with an electric mixer for 3 minutes.
Add vanilla extract to the egg mixture and mix in.
Add cocoa mixture and mix it in until smooth.
Add flour and salt (I recommend sifting in right over the cocoa mixture) and fold in with a rubber spatula.
Spread the batter in the prepared pan.
In a microwave, warm up the peanut butter in a heat-proof dish.
Drizzle the peanut butter in long, wide streaks over the brownie batter. Make swirls with a knife or offset spatula. Tap the pan against the counter to release all of the air bubbles that form on the surface.
Bake the brownies for 22 to 25 minutes or until the top if crackly and dry (the peanut butter will be melty).
Cool completely in the pan.
Remove the brownies from pan using the overhanging sides of the paper.