Preheat the oven to 350°F.
Grease a 13 x 9 inch baking pan with butter or non stick spray.
Mix the chocolate cake ingredients together as per the directions and bake for 20-25 minutes.
Let the cake cool for 5 minutes before poking lots of holes in the sponge with a thin wooden skewer or similar type of kitchen tool
Whisk the chocolate milk, condensed milk, evaporated milk and vanilla together until combined.
Slowly pour the liquid over the sponge and transfer to the fridge overnight or for at least 8 hours.
After the milk has fully soaked in to the sponge, whip the heavy cream and sugar together until soft peaks are formed.
Spread the cream over the top of the chocolate sponge.
Use a grater or vegetable peeler to shave the chocolate bar, and sprinkle the chocolate on top of the whipped cream.
Cut into squares and serve.