Preheat the oven to 350°F (175°C) and line a cupcake pan with cupcake liners.

In a mixing bowl, combine crushed graham crackers, brown sugar, and melted unsalted butter. Mix until crumbly.

Spoon about 1 tablespoon of the graham cracker mixture into each cupcake liner, pressing down gently to form a crust.

In a large bowl, whisk together all-purpose flour, sugar, cocoa powder, espresso powder, baking soda, baking powder, and salt.

In a separate bowl, mix eggs, buttermilk, warm water, vegetable oil, and vanilla extract until blended.

Gradually add wet ingredients to dry ingredients, stirring until just combined (do not overmix).

Pour chocolate cupcake batter over the graham cracker crust in each liner, filling about two-thirds full.

Bake for about 30 minutes, or until a toothpick inserted comes out clean.

Allow cupcakes to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.

In a small saucepan, heat heavy cream over medium heat until it simmers.

Remove from heat and add chopped dark chocolate. Let sit for a few minutes, then stir until smooth.

Allow the ganache to cool slightly before piping.

Once cupcakes are cooled, pipe a layer of chocolate ganache on top of each cupcake.

Spoon a dollop of marshmallow fluff on top of the ganache.

Lightly toast the marshmallow fluff with a kitchen torch or under a broiler (watch closely to avoid burning).

Let cool slightly before serving. Enjoy your delicious s'mores cupcakes!