Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add milk, vegetable oil, vanilla extract, and egg to the dry ingredients. Mix until well combined.
Fill each cupcake liner halfway with batter. Place a marshmallow in the center of each.
Top with remaining batter, covering the marshmallow completely.
Bake for 20-25 minutes, or until a toothpick inserted into the cupcake (but not the marshmallow) comes out clean.
While cupcakes are baking, mix graham cracker crumbs and melted butter in a small bowl.
Once cupcakes are done, let them cool for 10 minutes. Then, top each with graham cracker mixture.