Place egg whites in a large bowl, let stand at room temperature for 30 minutes. Sift 1 cup brown sugar and flour together twice; set aside. Sift remaining sugar; set aside.
Beat egg whites until foamy. Add cream of tartar and salt; beat until soft peaks form. Add vanilla. Gradually add reserved sugar, about 2 tablespoons at a time, beating well after each addition; beat until stiff glossy peaks form. Gradually fold in flour mixture, abut 1/2 cup at a time.
Gently spoon into an ungreased 10-in. tube pan. Bake at 350° for 40-45 minutes or until lightly browned and entire top appears dry.
Immediately invert pan onto a wire rack; cool completely, about 1 hour. Run a knife around side of cake and remove to a serving plate.