Heat oven to 350°F. Spray bottom only of 9x13 pan with baking spray with flour.
In large bowl, beat cake ingredients with electric mixer on low speed until moistened, then on high speed 2 minutes.
Add batter to pan and spread evenly. Bake 24 to 30 minutes or until toothpick inserted in center comes out clean.
Cool completely on a wire rack for about 30 minutes.
Meanwhile, in large bowl, beat cream cheese and butter with electric mixer on low speed until smooth. Gradually beat in powdered sugar, 1 cup at a time, until smooth.
Spread thin layer of frosting on side of cake. Spread remaining frosting on side and top of cake. Store covered in refrigerator