01 - Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until thoroughly mixed.
03 - In a large bowl, whisk the melted butter, light brown sugar, and granulated sugar until well combined and glossy. Add the eggs and vanilla extract, whisking until smooth.
04 - Gradually fold the dry ingredients into the wet mixture until just combined. Take care not to overmix.
05 - Gently fold in the chocolate chips or chopped chocolate, reserving a small handful for topping.
06 - Spread the batter evenly into the prepared pan. Dollop spoonfuls of creamy peanut butter over the top and swirl into the batter using a knife or skewer. Sprinkle with reserved chocolate.
07 - Bake for 23 to 26 minutes, or until the edges are golden and a toothpick inserted into the center comes out with a few moist crumbs.
08 - Allow to cool completely in the pan on a wire rack. Lift out using the parchment paper and cut into 16 squares.