Mix all cake ingredients together. Pour into sprayed 13x9 pan. Bake at 350 for 30-35 mins. Cake is done when center is firm and springs back when you lightly touch it. Cool completely before frosting.
Carefully mix frosting ingredients together and spread over cake. Keep cake refrigerated. Tastes even better after its been in the fridge for an extra day or two! I've tried this with lemon pudding as well and its very good.