Preheat oven to 350°F and line a 9x13 inch baking pan with parchment paper, leaving overhang on two sides. Lightly grease the paper.
In a large bowl, whisk melted butter and sugar for 2 minutes until glossy and well combined.
Add eggs one at a time, whisking well after each. Stir in vanilla extract, orange zest, and orange juice.
Sift cocoa powder, flour, salt, and baking powder into wet ingredients. Fold gently with a spatula until no dry streaks remain.
Fold in chocolate chips if using. Pour batter into prepared pan and spread evenly to corners.
Bake for 25-30 minutes until top is set and toothpick inserted in center comes out with moist crumbs.
Cool completely in pan on wire rack. Lift out using parchment overhang and cut into 16 squares.