Preheat oven to 350°F (175°C).
Prepare cake mix according to box directions.
Bake in 9x13 inch pan for 30-35 minutes.
While still hot, poke holes all over cake with wooden spoon handle.
Pour sweetened condensed milk over hot cake.
Pour caramel sauce over milk.
Sprinkle half the toffee bits over top.
Let cool completely (30-45 minutes).
Spread whipped topping evenly.
Top with remaining toffee bits and crushed Heath bars.
Drizzle with caramel sauce.
Refrigerate 2 hours before serving.