01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk together melted butter and sugar until well combined and glossy.
03 - Add eggs one at a time, whisking well after each addition. Stir in vanilla extract.
04 - In a separate bowl, sift together flour, cocoa powder, baking powder, and salt.
05 - Gently fold dry ingredients into wet mixture until just combined. Do not overmix.
06 - Fold chopped walnuts and chocolate into batter until evenly distributed.
07 - Pour batter into prepared pan and smooth top with spatula.
08 - Bake for 28 to 32 minutes, or until a skewer inserted in center comes out with a few moist crumbs.
09 - Remove from oven and let cool completely in pan before lifting out and cutting into squares.