Bake the chocolate cake in a 9×13-inch (23×33 cm) baking dish according to the package directions, using the additional ingredients called for on the box. Allow the cake to cool for at least 1 hour before adding the filling.
1 prepared 9×13-inch (23×33 cm) chocolate cake
Use the handle of a wooden spoon or a large skewer to poke holes evenly across the surface of the cooled cake, making sure the holes are wide enough for the hot fudge sauce to enter. Warm the hot fudge sauce until it is pourable but not hot, reserving about 2 tablespoons (30 ml) for garnishing, if desired. Pour the remaining fudge over the cake, directing it into the holes. Use an offset spatula or the back of a spoon to spread any fudge remaining on the surface and encourage it into the openings.
1 jar (12 oz / 340 g) hot fudge sauce
Refrigerate the fudge-filled cake, uncovered, for at least 2 hours, preferably 3 hours, until thoroughly chilled and the fudge has firmed up. Leaving the cake uncovered during this initial refrigeration prevents condensation from collecting beneath a cover and adding excess moisture to the cake. If refrigerating longer than 3 hours, cover the cake once it is cold.
Whisk the instant chocolate fudge pudding mix, cold whole milk, and mint extract together until smooth and beginning to thicken. Immediately spread the pudding evenly over the chilled cake, allowing it to fill any remaining openings. Instant pudding thickens quickly, so spread it as soon as it's mixed. Cover and refrigerate for at least 2 additional hours, or preferably overnight, to allow the pudding to absorb into the cake and the filling to become thoroughly chilled.
1 (3.9 ounces / 110 g) instant chocolate fudge pudding mix, 2 cups (480 ml) cold whole milk, ¼ tsp (1.25 ml) mint extract
Shortly before serving, beat the cold heavy whipping cream, confectioners' sugar, mint extract, and optional green food coloring in a chilled mixing bowl until stiff peaks form. The whipped cream should hold together when the beaters are lifted without becoming grainy.
1⅔ cups (400 ml) cold heavy whipping cream, ¾ cup (90 g) confectioners' sugar, ¼ tsp (1.25 ml) mint extract, 2-3 drops green food coloring
Spread the mint whipped cream evenly over the chilled cake. Sprinkle with chopped Andes mint chocolates and drizzle with the reserved hot fudge sauce, if desired. Slice into 12 large servings and serve cold. Refrigerate any leftovers.
½ cup (~85 g) chopped Andes mint chocolates, reserved hot fudge sauce