Preheat oven to 225 degrees F.

Using a 9-inch cake circle as a guide, with a pencil trace a circle on a sheet parchment paper cut to fit a baking sheet. Turn the paper over and with trace mark down, place on a baking sheet.

Place 4 egg whites, 1/8 teaspoon cream of tartar, and 1/8 teaspoon salt in the bowl of an electric mixer fitted with a balloon whip. Whisk on high speed until soft peeks form, about 45 to 50 seconds. Gradually add 1 cup sugar while continuing to whisk on high speed. 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 cup sugar, 2 tablespoons cocoa, and 1 tablespoon cornstarch. Fill a pastry bag (with no tip) with cocoa meringue. Fill the traced 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. Reduce heat to 200 degrees F. and bake for 2 hours and 45 minutes. Remove from the oven and allow the cocoa meringue to cool on baking sheet for 45 minutes before handling. Adjust the oven temperature to 325 degrees F.