Prepare cake in 13x9" pan according to box directions.
Use spoon handle to poke holes evenly across cake.
Combine milk and pudding mix and whisk until well blended.
Pour pudding over cake making sure it gets down into holes.
Refrigerate cake for several hours to allow pudding to settle and set up.
Open frosting container and remove foil seal.
Microwave for 15 seconds and stir. Repeat this until frosting is pour-able.
Pour over pudding layer and spread with spatula to cover completely.
Refrigerate for at least several more hours.