1Heat oven to 350°F (325°F for dark or nonstick pans). Grease bottom and sides of 13x9-inch pan or two 9-inch round cake pans and lightly flour, or spray with baking spray with flour.
2In large bowl, beat cake ingredients with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pan(s).
3Bake as directed on box. Cool rounds 10 minutes; remove from pans. Cool completely.
4In chilled large bowl, mix cream cheese and milk until smooth. Beat in whipping cream and powered sugar with electric mixer on high speed, scraping bowl occasionally, until soft peaks form. Frost top of 13x9-inch cake or fill and frost cake layers. Store loosely covered in refrigerator.