Preheat the oven to 350°F (175°C).
Grease a 9x9 inch baking pan or line it with parchment paper.
In a mixing bowl, combine melted butter and sugar.
Add eggs, one at a time, mixing well after each addition.
Stir in vanilla extract.
Sift together cocoa powder, flour, salt, and baking powder in a separate bowl.
Gradually add dry ingredients to the wet ingredients, mixing until just combined.
Fold in chocolate chips if using.
Pour batter into the prepared baking pan and spread evenly.
Bake for 20-25 minutes until a toothpick comes out with a few moist crumbs.