01 - Preheat the oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Over low heat in a medium saucepan, melt the butter with the chocolate chips, stirring until smooth. Remove from heat and let cool slightly.
03 - In a large bowl, whisk together granulated sugar, brown sugar, and eggs until mixture is thick and pale, about 2 minutes. Stir in vanilla extract.
04 - Pour the slightly cooled chocolate mixture into the egg mixture and stir until fully incorporated.
05 - Sift flour, cocoa powder, baking powder, and sea salt into the bowl. Fold gently until just combined; avoid overmixing.
06 - Fold the chopped walnuts gently into the batter.
07 - Pour the batter into the prepared pan and smooth the top evenly with a spatula.
08 - Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out with moist crumbs but no wet batter. Avoid overbaking.
09 - Allow brownies to cool completely in the pan on a wire rack. Use parchment paper handles to lift out, then cut into 16 squares and serve.