Normally when we make Jello Cake, we use a 9x13" pan and make it single layer. However, I was reminded that the original recipe called for it to be a two-layer cake. Then you can use two different colors/flavors of jello. Thanks, Peggy, for the birthday surprise!
Oh, and if you mix pudding into the Cool Whip for the frosting, that becomes more scrumptious as well.