Preheat oven to 350°F and grease an 8x8-inch baking pan.

Whisk melted butter and sugar together in a large bowl until combined.

Beat in eggs and vanilla until smooth.

Stir in cocoa powder, flour, salt, and baking powder until just combined — don't overmix.

Pour batter into prepared pan and bake for 20-25 minutes, until a toothpick comes out with moist crumbs.

Let cool completely before cutting into squares.