Let egg whites stand at room temperature 30 minutes. Sift flour and confectioners' sugar together twice; set aside. In a large bowl, beat the egg whites, cream of tartar and salt on medium speed until soft peaks form. Add vanilla and salt on medium speed until soft peaks form. Gradually add sugar, about 2 tablespoons at a time, beating on high until stiff glossy peaks form and sugar is dissolved. Gradually fold in flour mixture, about 1/2 cup at a time.
Gently spoon into an ungreased 10-in. tube pan. Bake at 325° for 50-60 minutes or until lightly browned and entire top appears dry. Immediately invert the pan onto a wire rack; cool completely.
For frosting, melt the candy bar in a heavy saucepan or microwave; stir until smooth. Cool for 5 minutes. In a chilled large bowl, beat cream until it begins to thicken. Add confectioners' sugar; beat until stiff peaks form. Stir in vanilla. Fold in melted chocolate.
Run a knife around sides and center tube of pan. Invert cake onto a serving plate; frost top and sides. Garnish with chocolate shavings if desired. Store in the refrigerator.