Beat in the eggs one at a time, ensuring each is well incorporated before adding the next.

Mix in the vanilla extract.

Add the dry ingredient mixture to the wet mixture gradually, alternating with the whole milk. Mix until just combined, beginning and ending with the dry ingredients.

Warm the peanut butter slightly in a microwave-safe bowl for about 15-20 seconds to make it easier to swirl. Stir until smooth.

Pour the brownie batter into the prepared baking dish and spread it out evenly.

Pour the warmed peanut butter on top of the brownie batter.

Swirl the peanut butter into the brownie batter gently using a knife or skewer, creating a marbled effect.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but not wet batter.

Remove from the oven and cool in the pan on a wire rack.

Cut into squares once completely cooled and serve.