Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch glass baking dish.

Mix cake mix, water, oil, and eggs in a large bowl with an electric mixer on medium speed until well combined. Pour batter into the prepared baking dish.

Bake in the preheated oven until a toothpick inserted in the center comes out clean, 30 to 35 minutes. Remove from the oven and set the baking dish on a wire rack.

While the cake is still very warm, poke holes all over the top with the end of a wooden spoon. Pour condensed milk over cake and let cool, 20 to 30 minutes.

Pour hot fudge topping over cooled cake and let set, 20 to 30 minutes.

Spread whipped topping over top and sprinkle with crushed toffee.