Unsweetened coco powder; for -dusting Confectioners sugar; -for dusting Preheat oven to 300. Line the bottom of a 9" springform pan with parchment paper. Lightly butter and flour the paper. Melt the chocolate and butter in a double boiler or metal bowl set over barely simmering water. Stir to combine, remove from the heat and set aside to cool. In a bowl, combine the egg yolks with all but 1 teaspoon of the sugar and beat at high speed until the mixture is light and forms a ribbon when the beaters are lifted from the bowl. In a separate bowl, combine the egg whites with the reserved teaspoon sugar. Beat until the egg whites form soft peaks. Fold one-third of the chocolate mixture into a bowl with one-third of the yolk mixture. Fold in one-third of the egg whites. Repeat two more times so that all of the ingredients are incorporated. Pour the batter into the prepared pan. Bake for 30 minutes. Do not overcook; the center of the cake should still be soft. Let the cake cool to room temperature for 3 hours; do not remove from the pan sooner or the cake will fall apart. Remove the sides of the springform pan and dust the cake with cocoa, then confectioners sugar and then cocoa and sugar again. Cover and refrigerate overnight. Before serving, let the cake warm to room temperature. Dust again with cocoa and confectioners sugar. Note: for best results, use dark bittersweet chocolate like Lindt or Tobler or can use semisweet chocolate and reduce the sugar to 1/2 cup.