Preheat the oven to 350°F (175°C) and line an 8x8-inch or 9x9-inch baking pan with parchment paper.
Whisk together the melted butter and granulated sugar until smooth.
Beat in the eggs one at a time, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Gradually fold the dry ingredients into the wet ingredients until just combined.
Fold in the chocolate chips and chopped nuts, if using.
Spread the batter evenly into the prepared baking pan.
Bake for 28–32 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.
Allow the brownies to cool completely in the pan.
Slice into 16 squares and serve.
