Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients and mix until well combined.
Carefully stir in the boiling water until the batter is smooth.
Fill each cupcake liner about ⅔ full with the batter.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely.
Top each cupcake with a large marshmallow and place under the broiler for 1-2 minutes, or until the marshmallows are golden brown.
Sprinkle graham cracker crumbs and chocolate chips on top of the toasted marshmallows.