Heat oven to 300 degrees. Grease a 10-inch springform pan with 1 tablespoon butter, and dust generously with granulated sugar.
Melt chocolate and butter over simmering water in a double boiler, about 6 to 8 minutes. Remove from heat and set aside to cool.
In an electric mixer fitted with the whisk attachment, whip yolks and whole egg with 2/3 cup sugar on a high speed, until it is pale yellow and falls in a thick ribbon when the beater is raised, about 5 minutes. Scrape down the sides of the bowl once or twice during mixing. Set aside.
In an electric mixer, using a second whisk attachment and bowl, (both should be clean and free of grease) whip egg whites with remaining 1/3 cup sugar on high speed until thick and foamy, about 3 to 4 minutes. Set aside. While egg whites are beating, add the cooled, melted chocolate slowly to the yolk mixture.
Temper whipped egg whites by mixing in about 1/4 cup of the chocolate mixture. Then gently fold egg whites into the chocolate yolk mixture, working quickly to maintain the volume of egg whites; the mixture will have a slightly marbleized appearance.
Pour batter into prepared pan, and place on a parchment-lined baking sheet. Bake in oven for 50 to 60 minutes. Remove from oven and allow to cool in the pan on a cooling rack. When completely cool, gently run a knife around the edges of the pan and release spring. Dust with cocoa powder or confectioners' sugar before serving.