1Heat oven to 350ºF. Grease and flour two 8- or 9-inch round cake pans, or 13x9-inch pan. 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 pans.
2Bake rounds 30 to 35 minutes, rectangle 35 to 40 minutes, or until toothpick inserted in center comes out clean. Cool rounds 10 minutes; remove from pans to cooling rack. Cool completely, about 1 hour.
3In medium bowl, mix powdered sugar and butter with spoon or electric mixer on low speed. Stir in vanilla and enough milk until smooth and spreadable. Fill and frost round layers or frost top of rectangle with frosting.