Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking dish.
In a large mixing bowl, combine the chocolate cake mix, water, vegetable oil, and eggs. Beat on medium speed for 2 minutes until well blended.
Pour the batter into the prepared baking dish and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Once the cake is baked, remove it from the oven and let it cool for about 15 minutes.
Using the handle of a wooden spoon, poke holes all over the top of the cake, about 1 inch apart.
In a separate bowl, mix the sweetened condensed milk and chocolate syrup until smooth. Pour the mixture evenly over the cake, allowing it to seep into the holes.
Let the cake cool completely, then refrigerate for at least 2 hours to allow the flavors to meld.
After chilling, spread the whipped topping over the cake. Sprinkle the crushed Heath bars on top for garnish.
Slice into squares and serve.