01 - Set oven temperature to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk melted butter with granulated and brown sugars until smooth and glossy.
03 - Whisk in eggs and vanilla extract until fully blended.
04 - Sift together flour, cocoa powder, salt, and baking powder. Fold gently into the wet mixture until just combined, avoiding overmixing.
05 - Gently fold chopped walnuts and optional chocolate chips into the batter.
06 - Pour batter into prepared pan and spread evenly with a spatula.
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 - Cool completely on a wire rack before lifting brownies out using the parchment overhang. Slice into 12 squares to serve.