Make cake mix according to package directions and bake in a well-greased 9×13 pan according to package.
Allow the cake to cool for about 10 minutes, then poke holes with a wooden spoon all over cake, about 1-inch apart.
Pour the sweetened condensed milk and 3/4 of the chocolate sauce over the top of the cake and allow it to seep into the holes that were poked into the cake.
Cover and refrigerate the cake overnight.
Place the heavy whipping cream, cocoa powder and powder sugar in a mixing bowl. Beat with an electric mixer for 4-5 minutes until stiff peaks form in the mixture and it holds it shape.
Spread the chocolate topping over the cake. Drizzle the top with the remaining chocolate sauce. Then sprinkle with the chopped chocolate.