Melt the chocolate over a hot water bath. Remove from the heat. Add the butter and sugar. Stir until the butter and sugar has melted. Separate the eggs. Stir the egg yolks, baking powder and flour into the chocolate mixture. Beat the egg whites until stiff and carefully fold into the batter. Line a baking sheet with parchment paper and pour in the batter.
Without preheating, bake the brownies for 40-50 minutes at 180°C (approximately 350°F), until a toothpick inserted into the center comes out clean. Remove from the oven and let cool on a wire rack. Cut into pieces and serve.