Cut challah bread into 1-inch square cubes and place into a large mixing bowl.
In a large measuring cup, whisk together milk, egg and cocoa powder.
Pour the liquid mixture on top of the bread, add the chocolate chips and marshmallows. Gently stir and make sure all the bread is covered with the liquid.
Using the room temperature butter, grease the insides of two 16-ounce ovensafe ramekins. Then add 1 tablespoon of graham cracker crumbs to each of the ramekins; move the ramekins around to completely coat the insides with crumbs.
Evenly divide the bread pudding mixture amongst the ramekins and top with more chocolate chips and marshmallows, if desired.
Place the ramekins on a large baking sheet. (The baking sheet is to help catch any overflow.)
Bake for 30 minutes, uncovered, on the middle rack in the oven. If your marshmallows start to turn too brown during the baking process, you can lightly cover with foil.
Let cool for 10 to 15 minutes before eating. Be careful — the middle is still really hot!
Notes
For fun, you can carefully use a kitchen torch and lightly toast the marshmallows on top even more for that campfire effect.