01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper leaving an overhang for easy removal.
02 - In a medium saucepan over low heat, melt the butter and chocolate chips together, stirring until smooth. Remove from heat and let cool slightly.
03 - In a large bowl, whisk granulated sugar, brown sugar, and eggs until thick and pale, about 2 minutes. Stir in vanilla extract.
04 - Pour melted chocolate mixture into the egg mixture and mix thoroughly until combined.
05 - Sift flour, cocoa powder, baking powder, and salt into the mixture. Fold gently until just combined without overmixing.
06 - Fold chopped walnuts into the batter evenly.
07 - Pour batter into prepared pan and smooth the surface with a spatula. Bake for 28 to 32 minutes until a toothpick inserted in the center comes out with moist crumbs.
08 - Cool completely on a wire rack. Lift brownies from pan using parchment overhang, cut into squares, and serve.