Preheat oven to 350°F and line an 8x8-inch baking pan with parchment paper.
In a large bowl, whisk together melted butter, granulated sugar, and brown sugar.
Add eggs and vanilla extract, whisking until smooth.
Stir in cocoa powder, flour, salt, and baking powder until just combined.
Spread the batter evenly into the prepared pan and bake for 20–22 minutes.
Remove from oven and let cool completely.