Preheat the oven according to the instructions on the chocolate cake mix box.
In a 9x13 inch baking dish, prepare the chocolate cake according to the package directions. Bake until a toothpick inserted in the center comes out clean, usually about 25 minutes. Let it cool completely before cutting into cubes.
In a mixing bowl, whisk together the chocolate pudding mix and milk according to package instructions, until smooth. Allow to set for about 5 minutes.
In another bowl, using an electric mixer, whip the cream until soft peaks form. Optionally, sweeten with a little sugar or vanilla extract to taste.
Begin assembling the trifle. In a trifle dish or individual cups, layer ⅓ of the chocolate cake cubes at the bottom.
Add a layer of ⅓ of the chocolate pudding on top of the cake.
Top the pudding with ⅓ of the whipped cream.
Repeat the layers two more times with the remaining cake, pudding, and whipped cream, finishing with a layer of whipped cream on top.
Garnish the top with chocolate shavings or chips and optional fresh berries.
Refrigerate for at least 2 hours before serving to allow the layers to set.