Preheat oven to 350°F (175°C) and grease an 8x8 inch square baking pan.

In a large bowl, combine melted butter and granulated sugar; whisk until well incorporated.

Add eggs one at a time, whisking thoroughly after each addition, then stir in the vanilla extract.

Sift together cocoa powder, flour, salt, and baking powder.

Gradually fold the dry ingredients into the wet mixture until just combined; do not overmix.

Stir in the chocolate chips if desired.

Spread the batter evenly into the prepared pan.

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

Allow to cool completely in the pan before cutting into squares.