First things first – preheat your oven to 350°F (175°C). Line your cupcake pan with cupcake liners.
In a mixing bowl, combine the crushed graham crackers, brown sugar, and melted butter. This is your crust! Spoon about 1 tablespoon of this mixture into each cupcake liner and press it down gently to form a firm base.
In a large bowl, whisk together the flour, sugar, cocoa powder, espresso powder (if using), baking soda, baking powder, and salt.
In another bowl, blend the eggs, buttermilk, warm water, vegetable oil, and vanilla extract. Gradually add the wet ingredients to the dry, stirring until just combined. Try not to overmix – we want fluffy, light cupcakes here!
Pour the chocolate cupcake batter over the graham cracker crust, filling each liner about two-thirds full. Bake for around 30 minutes, or until a toothpick inserted in the center comes out clean. Let the cupcakes cool in the pan for about 10 minutes, then transfer them to a wire rack to cool completely.
While your cupcakes are cooling, heat the heavy cream in a small saucepan over medium heat until it begins to simmer. Remove from heat and add the chopped dark chocolate. Let it sit for a few minutes, then stir until the chocolate is fully melted and smooth. Let the ganache cool slightly before piping it onto the cupcakes.
Once the cupcakes are fully cooled, pipe or spread a generous layer of chocolate ganache on top of each one. Next, add a heaping spoonful of marshmallow fluff on top of the ganache.
Now for the fun part! Grab your butane torch (or pop the cupcakes under a broiler, but watch carefully!) and lightly toast the marshmallow fluff until it’s beautifully golden. Be careful not to burn it, but a little bit of browning gives it that perfect s'mores flavor.
Let your cupcakes cool slightly before serving. Then, dive into the gooey, chocolatey goodness!