Prepare and bake the cake according to the package instructions in a greased 9x13" pan.
1 box white cake mix
While the cake is still hot, poke holes in the cake with the end of a wooden spoon or with a straw.
In a microwave-safe bowl, boil one cup of water. Stir in the orange gelatin until dissolved. Stir in one cup of the orange sherbet until melted.
1 3-ounce box orange gelatin, 1 cup boiling water, 2 cups orange sherbet
Pour over the cake slowly, allowing the mixture to soak into the holes; refrigerate for at least 4 hours.
In a large mixing bowl, melt the remaining cup of orange sherbet. Add the milk and pudding mix and whisk; let it set for a couple minutes to allow the pudding to thicken.
1 3.4-ounce box instant vanilla pudding mix, 1/4 cup milk
Add the whipped topping and fold in until smooth and well-combined.
1 8-ounce tub whipped topping
Spread over the cooled cake and garnish with mandarin oranges, if desired.
mandarin oranges, for garnish