Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a large bowl, whisk together the melted butter and granulated sugar until smooth and glossy. Add eggs one at a time, whisking well after each. Stir in vanilla extract.
In a separate bowl, sift together all-purpose flour, unsweetened cocoa powder, baking powder, and salt.
Gradually fold the dry ingredients into the wet mixture just until combined. Then fold in chocolate chips and walnuts if using.
Pour batter into prepared pan and spread evenly. Bake for 30-35 minutes until a toothpick comes out with a few moist crumbs.
Allow bars to cool completely in the pan on a wire rack. Lift out using parchment overhang, slice, and enjoy.