Prepare the chocolate cake mix according to the directions on the box.
Pour the cake batter into a greased 9-inch x 13-inch cake pan. Bake according to the time listed on the box.
While the cake is baking, place a medium saucepan over medium heat. Melt butter in the saucepan and then add the brown sugar and heavy whipping cream. Bring to a boil then remove from heat. Let the caramel sauce cool slightly.
Pour the caramel sauce into a medium mixing bowl and stir in sweetened condensed milk until evenly combined.
Remove the cake from the oven. While the cake is still warm, poke holes in the cake using the handle of a wooden spoon or a skewer.
Pour the caramel mixture over the top of the cake and spread it around evenly. Sprinkle the peanuts over the top of the cake.
Chill the cake in the fridge to cool for 1 hour.
Once the cake has cooled completely, remove the cake from the fridge and spread the Cool Whip over the top of the cake in an even layer. Drizzle with chocolate sauce and top with chopped Snickers pieces before cutting into 12 slices.