Bake cake according to package instructions. Allow to partially cool on a wire rack.
Poke holes with wooden skewer or end of a wooden spoon. For this cake, bigger holes are better so the chocolate-marshmallow mixture can sink in.
Melt chocolate chips together with marshmallows, and a tablespoon of butter on stovetop over low heat stirring continuously.
Stir in ¾ cup sweetened condensed milk.
Pour over the top of the cake, spreading around with a spoon or spatula 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 the remaining sweetened condensed milk and heavy whipping cream and mix together to incorporate.
Spread the chocolate cheesecake layer over the chocolate marshmallow layer.
Refrigerate for at least an hour prior to serving.
Decorate with graham crackers, chocolate bars and marshmallows prior to serving.