Preheat oven to 350 degrees F. In a large bowl, combine boxed cake mix, cocoa powder, pudding, milk, oil, eggs, sour cream, chocolate syrup and vanilla extract. Stir until mixed well. Pour into a greased 9 x 13 baking pan for about 20-25 minutes or until a toothpick into the center comes out clean. Allow to cool completely then using a wooden stir spoon, poke holes all over the cake; lots of them.
Combine sweetened condensed milk and chocolate chips in a microwave safe dish. Heat up for about 1 minute then remove and stir until chocolate chips are dissolved. This can be done on the stove top as well but heat up the sweetened condensed milk really well first, then remove from the heat and stir in chips. Stir in chocolate syrup, if desired. Gently pour mixture into hole using a measuring cup, this should take a few minutes. Refrigerate for an hour or more.
To make the frosting, add heavy cream, powdered sugar, cocoa and vanilla extract in a bowl then whip until stiff peaks form. Spread whipped topping evenly over cooled cake and allow frosting.
Dust cake with cocoa powder, crushed Oreos, sliced Reese's Peanut Butter cup candies, chopped nuts then drizzle with chocolate syrup.