12 ounces 60% chocolate
4 ounces (1/2 cup, or 1 stick) butter
2 tablespoons cocoa powder
4 egg yolks
6 egg whites
4 ounces sugar
Heat the oven to 375 degrees. In a bowl set over a pot of simmering water, melt together the chocolate and butter. Remove from the heat, then whisk in the cocoa powder and after that the egg yolks. Set aside.
In a medium bowl, whisk the egg whites and salt until foamy. Gradually add the sugar to the egg whites and whip to medium peaks.
Stir a third of the whites vigorously into the chocolate mixture until fully combined. Then fold in the remaining whites in two parts until all ingredients are incorporated.
Pour the mixture into a greased, parchment-lined 8-inch cake pan and bake until puffed and set, 18 to 20 minutes. (For individual cakes, use 4-inch greased brown paper molds. Fill each with 4.5 ounces batter; bake for 13 to 15 minutes.) Cool completely on a rack, then cover and refrigerate until chilled. Serve chilled, or bring to room temperature before serving.