Prep the pan and oven: Preheat your oven to 350°F (175°C). Lightly grease a standard 6-cavity doughnut pan with nonstick spray or melted butter. If you have two pans, you can bake 12 at once; otherwise, bake in batches.
Mix the dry ingredients: In a medium bowl, whisk together the flour, graham cracker crumbs, baking powder, baking soda, and salt until well combined.
Combine the wet ingredients: In a large bowl, whisk the brown sugar, granulated sugar, buttermilk, egg, melted butter, and vanilla until smooth and slightly frothy.
Make the batter: Add the dry ingredients to the wet ingredients. Stir with a spatula until just combined. The batter should be thick but scoopable. Do not overmix; a few small lumps are fine.
Fill the pan: Transfer the batter to a piping bag or a zip-top bag with the corner snipped. Pipe the batter into the doughnut cavities, filling each about 3/4 full. Alternatively, spoon it in carefully.
Bake: Bake for 9–12 minutes, until the doughnuts spring back when lightly pressed and a toothpick comes out clean. The tops should look set and lightly golden.
Cool: Let the doughnuts rest in the pan for 5 minutes, then turn them out onto a wire rack to cool completely before glazing.
Make the chocolate glaze: In a microwave-safe bowl, combine the chopped chocolate, butter, and corn syrup (if using). Microwave in 20–30 second bursts, stirring each time, until smooth. Add 2–3 tablespoons of milk, a little at a time, to reach a thick but pourable consistency. Stir in a pinch of salt.
Glaze the doughnuts: Dip the top of each cooled doughnut into the glaze, letting excess drip back into the bowl. Set them on the wire rack.
Add the s’mores topping: While the glaze is still tacky, press a small handful of mini marshmallows onto each doughnut and sprinkle with crushed graham crackers. If you have a kitchen torch, lightly toast the marshmallows until golden for that signature campfire flavor. No torch? Place glazed doughnuts (without the chocolate melting too much) under the broiler for 15–25 seconds—watch constantly.
Set and serve: Let the glaze set for 10–15 minutes. Serve slightly warm for the ultimate gooey bite, or at room temperature for easy handling.