01 - Set the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk together melted butter, granulated sugar, and brown sugar in a large bowl until smooth and glossy.
03 - Add eggs and vanilla extract to the wet mixture, whisking until fully blended.
04 - Sift flour, cocoa powder, salt, and baking powder into the wet mixture. Fold gently with a spatula until just combined, being careful not to overmix.
05 - Carefully fold chopped walnuts and chocolate chips into the batter if using.
06 - Pour the batter evenly into the prepared pan. Bake for 28 to 32 minutes or until a toothpick inserted in the center comes out with moist crumbs.
07 - Allow brownies to cool completely on a wire rack before lifting from the pan using the parchment overhang and slicing into twelve squares.