01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal after baking.
02 - In a large mixing bowl, whisk together the melted butter and granulated sugar until well combined and glossy in appearance.
03 - Add eggs one at a time to the butter mixture, whisking thoroughly after each addition. Stir in the vanilla extract until fully incorporated.
04 - In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt to remove lumps and aerate the dry mixture.
05 - Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Avoid overmixing to maintain a tender crumb structure.
06 - Fold the chopped walnuts and dark chocolate pieces into the batter until evenly distributed throughout.
07 - Pour the batter into the prepared baking pan and use a spatula to smooth the top surface into an even layer.
08 - Bake for 28 to 32 minutes, or until a skewer inserted in the center comes out with a few moist crumbs. Do not overbake.
09 - Remove from oven and allow brownies to cool completely in the pan. Lift out using parchment overhang and cut into 12 equal squares.