01 - Preheat oven to 350°F. Grease a 9x9-inch baking pan and line with parchment paper.
02 - Whisk together flour, cocoa powder, salt, and baking powder in a medium bowl.
03 - Combine melted butter and sugar in a large bowl. Whisk until smooth and glossy.
04 - Beat in eggs one at a time, mixing well after each addition. Stir in vanilla extract.
05 - Add dry ingredients to the wet mixture. Gently fold until just combined; avoid overmixing.
06 - Fold in chocolate chips and chopped walnuts until evenly distributed.
07 - Pour batter into the prepared pan and spread evenly with a spatula.
08 - Bake for 28-32 minutes until a toothpick inserted in the center comes out with a few moist crumbs.
09 - Let cool completely in the pan before cutting into 16 squares.