Preheat oven to 350°F. Coat the bottom only of a 13- x 9-inch baking dish or pan with cooking spray; set aside. Gather all ingredients.
Beat together cake mix, water, oil, and eggs in a large bowl with an electric mixer on medium speed until well combined, about 2 minutes. Spread batter evenly in prepared baking dish.
Bake in preheated oven until a wooden pick inserted in center comes out with a few crumbs attached, 28 to 30 minutes.
Meanwhile, whisk together sweetened condensed milk and caramel topping in a medium bowl until smooth; set aside.
While cake is still warm, use blunt end of a wooden skewer to poke holes all over surface of the cake at about ½-inch intervals. Slowly pour and spread condensed milk mixture over cake, making sure to fill all holes. Let cool completely in pan on a wire rack, about 1 hour. Cover and refrigerate until cool, at least 2 hours or up to 12 hours.
Remove cake from refrigerator. Drizzle with hot fudge topping, and sprinkle evenly with ½ cup of the toffee bits. Spread whipped topping evenly over top of cake, and sprinkle with remaining ¼ cup toffee bits. Garnish with chocolate shavings. Serve immediately or cover and refrigerate for up to 4 days.
