How to Make It
Preheat the oven to 400°. Grease eight 10-ounce ramekins with butter and dust with sugar, tapping out the excess; set on a rimmed baking sheet.
In a large bowl set over a saucepan of simmering water, melt the bittersweet chocolate with the 2 tablespoons of butter and the kosher salt, stirring a few times. In another medium saucepan, bring the milk just to a simmer over moderate heat. Whisk in the cocoa powder, then whisk in the melted chocolate.
In a large bowl, beat the egg yolks. Gradually whisk in the chocolate mixture until smooth. In another large bowl, using an electric mixer, beat the egg whites at high speed until medium peaks form, about 2 minutes. Gradually beat in the 1/2 cup of sugar and continue beating until the whites form stiff peaks, 2 to 3 minutes.
Carefully fold the egg whites into the chocolate mixture until no streaks remain. Spoon the soufflé into the prepared ramekins and bake in the center of the oven until risen, about 15 minutes. Serve right away.