Bake cake in 13x9 pan according to package instructions. Allow to cool.
Using a fork, poke holes in the top of the cake.
Pour gelatin powder into a medium bowl and scoop out two tablespoons to mix together with the cream cheese. With the remaining gelatin, mix together until dissolved with ¾ cup boiling water.
Carefully spoon the gelatin over the cake, making sure to pour over the holes.
Next, mix together cream cheese with 2 tablespoons gelatin until smooth. Add in the powdered sugar and milk.
Top with whipped topping. Chill for 4 hours before serving.