Position an oven rack in the middle of the oven and preheat the oven to 350 degrees F. Line a 9-inch square baking dish with foil and grease with cooking spray.
Melt the semisweet chocolate and butter in a medium heatproof bowl set over a saucepan of simmering water, stirring occasionally until smooth.
Beat together the sugar and eggs in a stand mixer on medium-high speed until creamed, about 3 minutes. Pour in the chocolate mixture and whisk until combined well. Mix in the flour, baking powder and salt for about 2 minutes on medium speed. Stir in the bittersweet chocolate chunks and transfer the batter to the prepared baking dish.
Bake until the top is shiny and set and the sides have begun to pull away slightly, about 35 minutes for fudgy brownies.