Preheat your oven to 350°F (175°C) and prepare a 9×13-inch baking pan by greasing it lightly. Combine cake mix with eggs, vegetable oil, and water as per package instructions, then pour into the prepared pan. Bake for 30-35 minutes until a toothpick comes out clean.

After baking, cool the cake in the pan for about 5 minutes. Poke holes all over the top with a skewer or wooden spoon handle, about 1 inch apart.

In a medium bowl, whisk together the hot chocolate pudding mix and cold milk until thickened but pourable, about 2 minutes.

Pour the pudding evenly over the poke holes in the cake and let absorb for a few moments.

Cover the cake with plastic wrap or foil and refrigerate for at least 2 hours or overnight.

After chilling, spread the whipped topping evenly over the surface of the cake.

Sprinkle mini marshmallows generously over the whipped topping.

Toast the marshmallows under a broiler or with a kitchen torch for about 1-2 minutes, keeping a close eye to prevent burning.

Drizzle chocolate sauce or fudge over the top before serving.