01 - Preheat the 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 mixing bowl, whisk granulated sugar, brown sugar, and eggs until thick and pale, about 2 minutes. Stir in the vanilla extract.
04 - Pour the melted chocolate mixture into the egg mixture and mix until well combined.
05 - Sift together the flour, cocoa powder, baking powder, and salt. Fold into the wet mixture gently until just combined, avoiding overmixing.
06 - Fold the chopped walnuts into the batter evenly.
07 - Pour the batter into the prepared pan and smooth the surface with a spatula.
08 - Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out with moist crumbs but no wet batter. Avoid overbaking.
09 - Cool completely on a wire rack in the pan. Use parchment overhang to lift out brownies, cut into squares, and serve.