Preheat your oven to 350 degrees. Prepare a 9-by-13-inch baking dish by spraying it with nonstick cooking spray. Set the dish aside.
In the bowl of a stand mixer fitted with a paddle attachment, combine the devil's food cake mix, eggs, milk, and butter until the mixture is smooth, about 2 minutes.
Add the sour cream, flour, and cocoa powder. Continue to mix the batter until it is smooth and the ingredients are combined.
Pour the batter into the prepared baking dish and place the dish in the oven for 35 minutes or until a toothpick comes out clean.
Once the cake is done baking, allow it to cool for 10 minutes. Using the back of a wooden spoon, poke several holes in the surface of the cake.
While the cake is cooling, add the sweetened condensed milk and caramel sauce to a medium saucepan over medium heat. Stir occasionally with the wooden spoon until the sauce has thinned, about 3-5 minutes.
Carefully pour the warm caramel sauce over the top of the cake. Then sprinkle the cake with 1 3/4 cups of the toffee bits.
Cover the cake with aluminum foil. Allow the cake to come to room temperature before placing the cake in the refrigerator to cool for at least 3 hours.
Once the cake is completely cool, spread the whipped cream topping over the cake with a silicone spatula. Garnish with the remaining 1/4 cup of toffee bits and drizzle with chocolate sauce.
Serve the cake chilled. Store the cake in the refrigerator.
As your cake cools, prepare the whipped cream. In the chilled bowl of a stand mixer fitted with a whisk attachment, combine the heavy whipping cream, sugar, and vanilla.
Whisk on high until the cream looks set and dense. Do not overmix; the mixture will begin to separate!