Heat oven to 350 degrees. In the bowl of an electric mixer fitted with a whisk attachment, beat egg whites and sugar on medium speed until foamy; add cream of tartar. Increase speed to high; beat egg whites until tripled in volume, stiff, and glossy. Transfer egg white mixture to a large bowl; fold in flour in three additions.
Spoon batter into an unbuttered 10-inch angel-food-cake pan with a removable bottom. Run a knife through batter to pop air pockets. Bake until cake is golden in color and springs back when gently pressed, about 45 minutes.
Invert pan onto a cooling rack or the neck of a bottle and allow cake to cool, at least 1 hour. To remove cake from pan, run a knife around sides and center tube of pan. Release cake from sides, and run a long knife along the bottom of the cake, freeing it from the tube. Serve with ice cream and hot-fudge sauce.
HOT FUDGE SAUCE: In a small saucepan over medium heat, bring cream and corn syrup just to a simmer. Remove from heat. Add chocolate; whisk until melted. Serve immediately. Sauce can be stored in an airtight container in the refrigerator up to 1 week.