Spray a 9x13” glass baking dish with cooking spray.
Mix cake mix, whipping cream, eggs and canola oil with an electric mixer at low speed about a minute.
Increase speed and beat another couple of minutes until cake texture is smooth and thick.
Spread cake batter into the bottom of prepared dish.
Bake at 325° for approximately 35-40 minutes or until a toothpick inserted in center comes out clean.
(Mine took 50 minutes.)
Allow cake to cool completely.
Poke holes in cake with a straw.
Pour a can of sweetened condensed milk over top of cake so that it oozes into the holes.
Spread with a knife if necessary to cover entire surface.
Allow cake to sit about 5 minutes.
Remove lid from hot fudge sauce.
Microwave about 45 seconds at 50% power or until of a spreadable consistency.
Very gently, spread about ¾ of the jar of hot fudge sauce over top of the cake and smooth over top with a knife so that the fudge sauce also goes into the holes.
Spread the container of Cool Whip over top of the cake.
If hot fudge sauce has gotten thick, microwave another 10-15 seconds at 50% power until it’s softer.
Drizzle remaining sauce over top of Cool Whip.
Add chopped Heath bars and sprinkle over top of the hot fudge sauce.
Cover and refrigerate.