01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl.
03 - In a large bowl, whisk melted butter and sugar until smooth and glossy. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
04 - Fold dry ingredients into the wet mixture gently using a spatula until just combined, avoiding overmixing.
05 - Carefully fold in chocolate chips and three-quarters of the chopped walnuts.
06 - Pour batter evenly into the prepared pan and smooth the surface. Sprinkle remaining walnuts evenly over the top.
07 - Bake for 28 to 32 minutes or until a toothpick inserted in the center comes out with moist crumbs but no wet batter.
08 - Allow to cool completely in the pan before using parchment overhang to lift out and cut into 16 squares.