Preheat oven to 350°F (175°C). Line cupcake tin with liners.
If making graham crust, mix crumbs, sugar, and butter. Press ~1 tbsp into each liner and bake for 5 minutes.
In a bowl, whisk flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In another bowl, beat oil, eggs, vanilla, and buttermilk until combined.
Mix wet and dry ingredients until just combined. Slowly mix in hot coffee or water.
Fill liners 2/3 full and bake 16–18 minutes. Cool completely.
Choose marshmallow topping method. Pipe or spread on cooled cupcakes.
Torch marshmallow or broil briefly for golden top.
Sprinkle with crushed graham crackers and add any other desired toppings.