Preheat the oven according to the cake mix package directions. Spray a 9x13-inch baking dish with nonstick cooking spray.

Prepare the cake batter using the cake mix, water, eggs, and oil according to the package directions. Pour the batter into the prepared baking dish and bake as directed.

Remove the cake from the oven and allow it to cool completely in the pan.

Using the handle of a wooden spoon, poke holes evenly across the top of the cake, stopping about three-quarters of the way down.

In a large mixing bowl, whisk together the instant vanilla pudding mix and milk until smooth and slightly thickened.

Pour the pudding mixture over the cake, spreading it evenly and gently pressing it into the holes.

Refrigerate the cake for 1 hour or until the pudding layer is set.

Remove the lid and foil seal from the chocolate frosting. Microwave in 15-second intervals, stirring after each interval, until the frosting is smooth and pourable.

Pour the frosting over the chilled pudding layer and spread it into an even layer.

Refrigerate for at least 1 additional hour before slicing and serving.