Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking dish or line it with parchment paper.
Whisk together the flour, cocoa powder, and salt in a medium bowl. Set aside.
Cream together the softened butter and sugar in a large mixing bowl until light and fluffy.
Beat in the eggs one at a time, ensuring each is well incorporated before adding the next.
Mix in the vanilla extract and 2 tablespoons of milk.
Add the dry ingredients to the wet mixture and mix just until combined, don’t overmix.
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 fo 30- 35 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.