Prepare cake per instructions on back of cake mix box for a 9 x 13 baking dish. Cool completely. (NOTE: for a more bakery flavored cake substitute the same amount of milk in place of the water and substitute melted butter for the oil.)
Using a straw or the rounded back end of a wooden spoon, poke lots of holes all the way through the cake to the bottom of dish. (NOTE: you need big holes in order for the pudding to fill in the holes.) Set a side.
In a large bowl place pudding, milk, and vanilla. Using electric hand mixer, beat until a loose pudding consistency is met.
1) you can pour pudding all over cake and using a spoon or other utensil push the pudding into holes of cake. OR2) spoon pudding into a piping bag or ziploc bag and pipe the pudding into holes filling the holes from the bottom up. Refrigerate cake for 1 hour for pudding to set. Start the chocolate step after the 1 hour is over.
In a small sauce pan over medium heat add heavy whipping cream. Stir cream until it starts to simmer along the edges. Turn off heat.
In a medium bowl place chocolate chips and butter. Pour heavy whipping cream over chocolate chips and butter while stirring carefully. Stir until chips and butter have melted and you have a nice smooth consistency. Cool for only 10 minutes.
Spoon chocolate over cake. (NOTE: chocolate sauce will SET pretty fast. Make sure to spoon the sauce where you want it as you do not want to spread it afterwards.)
Refrigerate for 4 hours to overnight to allow the chocolate to set. Cut into squares and enjoy.