1Heat oven to 350°F. Grease and flour bottom and sides of 13×9-inch pan, two 9-inch round cake pans, or three 8-inch round cake pans. In large bowl, beat all cake ingredients with electric mixer on low speed 30 seconds, scraping bowl constantly. Beat on high speed 3 minutes, scraping bowl occasionally. Pour into pan(s).
2Bake rectangle 40 to 45 minutes, rounds 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool rectangle in pan on cooling rack. Cool rounds 10 minutes; remove from pans to cooling rack. Cool completely.
3In 2 1/2-quart saucepan, mix all frosting ingredients except vanilla. Heat to rolling boil, stirring occasionally. Boil 1 minute without stirring. Place saucepan in bowl of ice and water. Beat until frosting is smooth and spreadable; stir in vanilla. Frost rectangle or fill and frost layers with frosting.