Step 1Heat 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.
Step 2In a medium bowl, whisk the egg whites and salt until foamy. Gradually add the sugar to the egg whites and whip to medium peaks.
Step 3Stir 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.
Step 4Pour 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.