Double Chocolate Brownies

Preheat oven to 350 degrees F. Line a buttered 8-inch square baking pan with foil or parchment paper, allowing 2 inches to hang over sides. Butter lining (excluding overhang); set pan aside. Put butter, chocolate, and cocoa in a heatproof medium bowl set over a pan of simmering water; stir until butter and chocolate are melted. Let cool slightly. Whisk together flour, baking powder, and salt in a separate bowl; set aside. Put sugar, eggs, and vanilla in the bowl of an electric mixer fitted with the whisk attachment, and beat on medium speed until pale, about 4 minutes. Add chocolate mixture; beat until combined. Add flour mixture; beat, scraping down sides of bowl, until well incorporated. Pour batter into prepared pan; smooth top with a rubber spatula. Bake until a cake tester inserted into brownies (avoid center and edges) comes out with a few crumbs but is not wet, about 35 minutes. Let cool slightly in pan, about 15 minutes. Lift out brownies; let cool completely on a wire rack before cutting into squares.