Preheat the oven to 375°F (190°C) and lightly grease a doughnut pan.
In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In another bowl, mix buttermilk, egg, melted butter, and vanilla until smooth.
Combine wet and dry ingredients just until blended.
Spoon the batter evenly into the prepared doughnut pan.
Bake for 8–10 minutes or until the doughnuts spring back when touched.
Let them cool in the pan for 5 minutes, then transfer to a wire rack.
Meanwhile, melt the chocolate chips in a microwave-safe bowl in 30-second increments until smooth.
Dip the tops of cooled doughnuts into the melted chocolate.
Sprinkle with mini marshmallows and crushed graham crackers.
Let the chocolate set before serving.