Preheat oven to 350°F (175°C). Lightly coat a 10-inch angel food cake pan with non-grease/non-stick spray
Sift cake flour, salt, and cream of tartar through a fine-mesh sieve 4 times until very light and fluffy
In a large bowl with chilled attachments, add egg whites. With mixer on medium speed, gradually add half the sugar (3/4 cup) for the cake meringue. Incorporate cream of tartar after whites form soft peaks
Increase speed and whip until sugar dissolves and meringue holds stiff peaks
Gently fold in 4 cups raspberries into the meringue to preserve air
With a spatula, fold meringue back into the sifted flour mixture in two additions, ensuring no collapse
Pour batter into prepared pan and spread evenly into a mound with an offset spatula
Bake 40 minutes or until golden and firm. Immediately invert pan to cool or cake will collapse
While cake cools, prepare optional meringue topping by whipping 12 refrigerated egg whites and 3/4 cup sugar until stiff peaks
Whisk in 1 teaspoon cream of tartar during whipping for maximum stability
Once completely cooled, loosen cake from pan, transfer to serving plate
Optional: Spread remaining 2 cups raspberries with 2 tablespoons cornstarch and 1/4 cup sugar in a saucepan until thickened. Let cool before layering
For topping, whisk optional meringue until stiff peaks and pipe around the cake or serve separate
Chill before serving to set meringue and shine