Preheat the oven to 225 degrees. Using a 9-inch cake circle as a guide, trace a circle with a pencil on a sheet of parchment paper cut to fit a baking sheet. Turn the paper over and, with trace mark down, place on a baking sheet. To prepare the cocoa meringue, place 4 egg whites, the cream of tartar, and salt in the bowl of an electric mixer fitted with a balloon whip. Whisk on high until soft peaks form, about 45 to 50 seconds. Gradually add 1 cup sugar while while continuing to whisk on high. Whisk until stiff, about 1 1/2 minutes. Remove the bowl from the mixer and use a rubber spatula to fold in and thoroughly combine the remaining 1/4 c sugar, 2 T cocoa, and the cornstarch. Fill a pastry bag (with no tip) with the cocoa meringue. Fill the traced parchment circle with meringue: start in the center and pipe a 3/4-inch-wide spiral towards the outside of the circle. Place the meringue in the preheated oven and bake for 15 minutes. Lower the oven temperature to 200 degrees and bake for 2 hours and 45 minutes. Remove from the oven and allow to cool for 45 minutes before handling. Raise the oven temperature to 325 degrees. While the meringue is baking, prepare the chocolate mousse. Heat 1 inch of water in the bottom half of a double boiler over medium heat. Place 6 ounces semi-sweet chocolate in the top half of the double boiler. Tightly cover the top with film wrap. Allow the chocolate to melt slowly, about 9 to 10 minutes. Remove from the heat and stir until smooth. Keep at room temperature until needed. Place 1 1/2 c heavy cream in the well-chilled bowl of an electric mixer fitted with a well-chilled balloon whip. Whisk on high until peaks form, about 1 minute. Set aside. Whisk 3 egg whites in a large stainless steel bowl, until soft peaks form, about 3 minutes. Add 2 T sugar and continue to whisk until stiff peaks form, about 2 to 2 1/2 minutes. Add a quarter of the whipped cream to the chocolate and whisk quickly, vigorously, and thoroughly, then add to the egg whites. Now add the remaining whipped cream. Fold all together gently but thoroughly. Refrigerate the chocolate mousse until needed. See Part 3