Make the brownie layer: Melt chocolate and butter together in a heatproof bowl over a double boiler. (You can also do this in the microwave in 15-second bursts, checking often to make sure the chocolate doesn’t burn.) Whisk together flour, baking powder, salt, and cocoa powder in another bowl. Transfer melted chocolate mixture into the bowl of an electric mixer fitted with the paddle attachment. Add sugar and beat on medium speed then add eggs one at a time. Turn speed to low then mix in flour mixture just until combined. Do not over-mix.