Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper or lightly grease it with butter or cooking spray. This makes it much easier to remove the brownies later.

In a medium mixing bowl, whisk together the melted butter and granulated sugar until smooth. Add the eggs and vanilla extract, then whisk again until the mixture becomes glossy and well combined.

Sift in the cocoa powder, flour, and salt. Gently stir everything together with a spatula until just combined. Be careful not to overmix, as that can make the brownies dense.

If you’re using chocolate chips, fold them into the batter at this stage. They add extra bursts of chocolate in every bite.

Spread the brownie batter evenly into the prepared baking pan using a spatula.

Drop small spoonfuls of peanut butter across the surface of the batter. Use a knife or toothpick to gently swirl it through the brownie batter to create a marbled effect.

Place the pan in the oven and bake for 20–25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Allow the brownies to cool completely in the pan before slicing. This helps them set properly and makes cutting cleaner squares much easier.