Preheat oven to 350°F and prepare your 8 x 8" baking pan with parchment paper.
Grab a large bowl and mix melted butter and sugar until well combined.
Beat in the eggs one at a time, then add the 1 tsp of vanilla extract
In a separate bowl, add in the All-Purpose Flour, cocoa powder, baking powder, and salt. Whisk together.
Gradually add these dry ingredients to the wet ingredients from Step #2. Use a handheld mixer, stand mixer, or a large spoon to mix. Mix until just combined, do not overmix.
Pour the brownie batter into the prepared baking pan. Spread the batter until it is level.
Grab a separate mixing bowl, add in the peanut butter, powdered sugar, the remaining 1 tsp vanilla extract, and the ¼ cup of milk. The milk helps make the mixture a thinner and easier to work with.
Drop spoonfuls of the peanut butter mixture onto the top of the brownie batter. Use a knife to create a marbled effect (see video for more detail). Use until the peanut butter mixture is mixed throughout.
Bake 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the brownies to cool for 5-10 minutes, then cut into squares. You can cut into 12 standard-sized brownies. Enjoy your brownies!