01 - Preheat oven to 350°F. Grease a 9x9-inch baking pan and line with parchment paper, allowing overhang on sides for easy removal.
02 - In a medium bowl, whisk together flour, cocoa powder, salt, and baking powder until well blended.
03 - In a large bowl, combine melted butter and sugar. Whisk vigorously until mixture becomes smooth and glossy, about 2 minutes.
04 - Beat in eggs one at a time, mixing thoroughly after each addition until fully incorporated. Stir in vanilla extract.
05 - Add dry ingredients to wet mixture. Gently fold using a spatula until just combined, being careful not to overmix. The batter should be thick and glossy.
06 - Fold in chocolate chips and chopped walnuts until evenly distributed throughout the batter.
07 - Pour batter into prepared pan. Spread evenly using an offset spatula, ensuring the surface is level.
08 - Bake for 28 to 32 minutes. Check for doneness by inserting a toothpick into the center; it should emerge with a few moist crumbs but not wet batter.
09 - Let brownies cool completely in the pan on a wire rack. Once cooled, use parchment paper overhang to lift out, then cut into 16 squares.