Prepare all ingredients in advance for a smooth cooking process.

Start by preheating the oven to 350°F (175°C). Lightly grease a 9-inch square glass baking pan to prevent sticking.

In a large mixing bowl, combine the brownie mix, egg, ¼ cup of water, and vegetable oil. Stir briskly until the mixture is fully combined and smooth, about 50 strokes. Spread this batter evenly in the prepared baking pan.

Bake the brownie mixture in the preheated oven for 25 to 30 minutes, or until a toothpick inserted about an inch from the pan’s edges emerges clean. Once baked, set the pan aside to cool completely—this will take around 30 minutes.

After cooling, cut the brownies into 1-inch squares and set them aside.

In another large bowl, mix together the condensed milk, the remaining ½ cup of water, and the chocolate pudding mix until smooth and thick. Gently fold in the 8-ounce container of thawed whipped topping until no streaks remain, creating a rich pudding mixture.

To assemble the trifle, place half of the brownie squares at the bottom of a trifle dish or a large glass serving bowl. Spread half of the chocolate pudding mixture over the brownies, then add a layer using half of the 12-ounce container of whipped topping. Repeat these layers with the remaining brownies, pudding mixture, and whipped topping.

For garnish, shave the chocolate candy bar on top of the trifle, adding a finishing touch of chocolate.

Refrigerate the assembled trifle for at least 8 hours before serving, allowing the flavors to meld and the dessert to set for optimal texture and taste.