Preheat the oven to 350°F (175°C).

Grease an 8x8 inch baking pan with butter or line it with parchment paper.

In a mixing bowl, combine the melted butter and granulated sugar. Whisk until smooth.

Add eggs one at a time, mixing well after each addition, followed by the vanilla extract.

In another bowl, whisk together cocoa powder, flour, salt, and baking powder.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in chocolate chips if using.

Pour the batter into the prepared pan and spread evenly.

Bake for 20 to 25 minutes or until a toothpick inserted comes out with a few moist crumbs.