In a large mixing bowl, combine melted butter and granulated sugar, then whisk until smooth. Beat in the eggs one at a time, followed by the vanilla extract. In a separate bowl, sift together flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in chocolate chips if desired.

Pour the batter into a greased 9x13 inch baking pan. Preheat your oven to 350°F (175°C) and bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Allow the brownies to cool completely in the pan on a wire rack before cutting them into squares. Serve plain or with a scoop of ice cream.