In electric mixer, beat egg whites until foamy on medium setting. Add remaining ⅓ cup sugar and cream of tartar. Beat on high until it is thick, tripled in volume until
medium peaks form. Add flour mix in three stages. Pour into ungreased Angel Food Cake pan. Bake at 350 degrees for 35 minutes until golden. To unmold, invert pan. Cool completely. Loosen with knife to remove.
Make the frosting
Set up a double boiler on the stove, making sure water does not touch the bowl, otherwise frosting will become grainy.
Combine all ingredients, except vanilla, in a stainless steel bowl.
Beat with electric mixer 3 minutes on low speed. Beat on high 7 minutes until stiff peaks form. Fold in vanilla. Ice angel food cake.