Preheat the oven: Preheat your oven to 350°F (175°C). Line a 12-cup muffin pan with liners or grease lightly.
Mix the dry ingredients: In a large bowl, whisk together the flour, crushed graham crackers, cocoa powder, sugar, baking powder, baking soda, and salt.
Whisk the wet ingredients: In a separate bowl, whisk together the eggs, buttermilk, oil, and vanilla extract.
Combine: Pour the wet ingredients into the dry and stir until just combined. Do not overmix.
Fold in mix-ins: Gently fold in chocolate chips and mini marshmallows.
Fill the muffin cups: Divide the batter evenly into the muffin cups, filling about ¾ full. Top each muffin with a few more mini marshmallows, chocolate chunks, and a sprinkle of crushed graham crackers.
Bake: Bake for 16–18 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (avoid overbaking to keep them soft).
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Serve: Enjoy warm or at room temperature for maximum gooey marshmallow goodness.