Preheat oven to 350°F. Line two standard 12-cup muffin pans with cupcake liners.
Pulse graham crackers in a food processor until they form fine, even crumbs. In a medium bowl, stir crumbs together with melted butter, sugar, and salt until mixture looks like wet sand.
Spoon about one tablespoon of crust mixture into each liner and press down firmly using the back of a spoon or the bottom of a small glass. Bake for 5-6 minutes, then remove from oven and set aside to cool slightly. Leave oven on.
In a large bowl, whisk together flour, Dutch process cocoa powder, sugar, baking powder, baking soda, and salt until no clumps remain.
In a separate large bowl, whisk together eggs, milk, vegetable oil, and vanilla until smooth. Whisk in hot coffee last (mixture will be thin).
Pour wet ingredients into dry ingredients and whisk just until no streaks of flour remain, being careful not to overmix. Batter will be thin.
Divide batter evenly over baked crusts, filling each liner about two-thirds full. Bake for 16-18 minutes, until a toothpick inserted in center comes out clean. Cool in pans 10 minutes, then transfer to wire rack to cool completely.
In a stand mixer, beat butter on medium speed until smooth and pale, about 2 minutes. Add marshmallow fluff and beat until combined. Gradually add powdered sugar on low speed to avoid a sugar cloud.
Add vanilla and heavy cream, then increase speed to medium-high and beat for 2-3 minutes until light and fluffy.
Transfer frosting to a piping bag fitted with a large star tip and pipe a generous swirl onto each cooled cupcake.
Use a kitchen torch to lightly toast the outside of the frosting until golden in spots. Finish with a sprinkle of graham cracker crumbs and a small piece of chocolate or mini marshmallow.