01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for removal.
02 - Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl.
03 - Whisk melted butter and sugar in a large bowl until smooth and glossy. Add eggs one at a time, beating well after each addition, then stir in vanilla extract.
04 - Fold dry ingredient mixture into the wet mixture using a spatula, mixing until just combined without overmixing.
05 - Gently fold in chocolate chips and three-quarters of the chopped walnuts.
06 - Pour batter into prepared pan, smooth top, and sprinkle remaining walnuts evenly over the surface.
07 - Bake for 28 to 32 minutes until a toothpick inserted in the center comes out with moist crumbs but no wet batter.
08 - Allow brownies to cool completely in the pan before lifting out and cutting into 16 squares.