1. Combine sugar, water, cream of tartar and salt in saucepan.
2. Bring to boiling, stirring until sugar dissolves.
3. Very slowly add sugar syrup to unbeaten egg whites in mixing bowl, beating constantly with electric mixer until stiff peaks form (about 7 minutes).
4. Beat in vanilla.
5. Frosts tops and sides of two 8 or 9-inch layers, or one 10-inch tube or one 9x13 pan.