01 - Preheat the oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang to facilitate removal.
02 - In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth and glossy.
03 - Add eggs and vanilla extract to the mixture and whisk until fully combined.
04 - Sift together flour, cocoa powder, salt, and baking powder. Fold the dry ingredients gently into the wet mixture using a spatula until just combined, avoiding overmixing.
05 - Fold in chopped walnuts and optional chocolate chips evenly throughout the batter.
06 - Pour batter into the prepared baking pan and spread evenly with a spatula.
07 - Bake for 28 to 32 minutes until a toothpick inserted into the center comes out with moist crumbs but no wet batter.
08 - Allow the brownies to cool completely in the pan placed on a wire rack before lifting out and slicing into 12 squares.