Preheat oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish or line it with parchment paper.
Combine wet ingredients: In a large mixing bowl, whisk together melted butter, granulated sugar, and brown sugar until smooth. Add eggs one at a time, whisking well after each addition. Stir in vanilla extract.
Combine dry ingredients: In a separate bowl, whisk together flour, unsweetened cocoa powder, baking powder, and salt. Gradually stir dry ingredients into wet mixture until just combined.

Fold in extras: If using, fold in chocolate chips and chopped nuts for extra texture and flavor.
Bake the brownies: Pour brownie batter into the prepared baking dish, spreading it out evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not completely clean for fudgy brownies).
Cool and serve: Let brownies cool in the pan for about 15-20 minutes before cutting them into squares. Serve and enjoy!