Bake cake according to package instructions. Allow to partially cool on a wire rack.
Poke holes with a wooden skewer or end of a wooden spoon. For this cake, the bigger the holes the better.
Melt chocolate chips together with marshmallows, and tablespoon of butter on the stovetop over low heat, stirring continuously.
Stir in ¾ cup of condensed milk.
Pour over the top of the cake, spreading around with a spoon to push the chocolate marshmallow mix into the holes.
For the next layer, cream together cream cheese, powdered sugar and cocoa powder with a hand mixer until smooth.
Add in remaining sweetened condensed milk and heavy whipping cream. Mix together.
Spread the chocolate cheesecake layer of the chocolate marshmallow layer.
Refrigerate for at least an hour prior to serving.
Decorate with graham crackers, chocolate bars, and marshmallows before serving.