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 mixing bowl, whisk together the melted butter and sugar until well combined and glossy.
03 - Add eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract.
04 - In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt.
05 - Gently fold the dry ingredients into the wet mixture until just combined, avoiding overmixing.
06 - Fold in the chopped walnuts and dark chocolate until evenly distributed throughout the batter.
07 - Pour the batter into the prepared pan and smooth the top with a spatula for even baking.
08 - Bake for 28 to 32 minutes, or until a skewer inserted in the center comes out with a few moist crumbs.
09 - Remove from oven and let cool completely in the pan before lifting out and cutting into 12 squares.