Preheat the oven to 350 degrees F. Spray a 9x13-inch baking dish with non-stick cooking spray, set aside.
Place the cake mix, water, oil, and eggs into a large bowl. Mix together with an electric hand mix for 30 seconds until combined. Beat for 2 minutes until smooth.
Pour into the baking dish and bake for 25-30 minutes until a toothpick inserted into the center comes out clean. Let cool on a wire rack completely.
In a large bowl, whisk together the instant vanilla pudding with the milk until thick, 2 minutes. Cover with plastic wrap and place in the refrigerator until ready to assemble.
Place the chopped chocolate in a medium-sized heat-proof bowl, set aside.
Place the heavy cream in a microwave-safe bowl. Heat the cream, 15 seconds at a time in the microwave until it reaches 180°F. Pour the hot cream over the chopped chocolate and let it sit for 5 minutes. Carefully whisk until smooth. Let this mixture cool to room temperature.
When you are ready to assemble, cut the cake into ½ inch to 1-inch cubes. Place ⅓ of the cake cubes into the bottom of a standard-sized trifle dish.
Top with ⅓ of the pudding.
Then top with ⅓ of the ganache.
Repeat two more times, saving a small amount of the ganache for garnish.
Smooth out the whipped topping on top. Drizzle the remaining ganache on top of the whipped topping. Serve immediately.