Preheat oven to 375 degrees F. Grease large muffin tins or arrange large silicone muffin cups on a baking sheet.
In a large bowl, whisk together the flour, graham cracker crumbs, brown sugar, baking soda, and salt. Mix in the egg, buttermilk, and vegetable oil to form a smooth batter. Fold in 3/4 cup of the chocolate chips and 1 cup of the marshmallows.
Fill the prepared muffin tins 3/4 full, then add the remaining 1/4 cup marshmallows and chocolate chips to the tops of the batter.
Bake in preheated oven until golden, the marshmallows on the top are bubbly, and toothpick inserted in the center comes out dry, 18-20 minutes.
Best served within a few hours of baking.