Empty your brownie mix into a bowl. Add the egg, water, and vegetable oil. You can whisk it by hand if you’re in the mood for a little arm workout, or let the electric mixer do the heavy lifting.
Line your 8×8 tin with parchment paper. I like to leave a little overhang on the sides so I can lift the whole brownie block out later without fighting with the corners of the pan.
Pour the brownie batter into your lined tin, spreading it evenly with a spatula. Now comes the fun part drizzle your peanut butter right over the top. Use as much or as little as you like. I usually go for at least 2 big spoonfuls, sometimes even 4, depending on how indulgent I’m feeling.
Take a butter knife and gently swirl it through the batter to create those beautiful marbled ribbons of peanut butter.
Pop your tin into a preheated oven at 320°F (160°C) and bake for 15–18 minutes. The key is to check with a toothpick if it comes out clean or with just a few moist crumbs, you’re good to go. Don’t overbake or you’ll lose that fudgy magic.
Let your brownies rest on a cooling rack for at least 10–15 minutes. This part is always torture because the house smells incredible, but it’s worth the wait cutting into hot brownies often leads to a gooey mess. Once cooled, lift them out using your parchment paper, slice into squares, and enjoy.