Preheat your oven to 350°F (175°C). Line a muffin tin with cupcake liners and set aside.

In a bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until well combined. Press about 1 tablespoon of the mixture into the bottom of each cupcake liner. Bake for 5 minutes, then remove from oven and let cool.

In a large mixing bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt. In another bowl, beat together granulated sugar, brown sugar, and vegetable oil until combined. Add eggs one at a time, beating well after each. Stir in vanilla extract. Gradually add the dry ingredients to the wet ingredients, alternating with buttermilk, starting and ending with the dry ingredients.

Divide the batter evenly among the cupcake liners, filling each about two-thirds full. Bake in the preheated oven for 18-22 minutes or until a toothpick inserted into the center comes out clean. Allow cupcakes to cool completely on a wire rack before proceeding.

In a small saucepan over low heat, combine chocolate chips and heavy cream. Stir continuously until the chocolate is melted and smooth. Let it cool slightly before using.

Once the cupcakes are cool, use a cupcake corer or a small knife to remove the center of each cupcake. Fill each with a spoonful of chocolate filling.

In a large bowl, beat the butter until creamy. Add marshmallow fluff and vanilla extract and beat until smooth. Gradually add powdered sugar, beating until the frosting is light and fluffy.

Pipe or spread the marshmallow frosting onto the filled cupcakes. Optionally, use a kitchen torch to lightly toast the frosting for a more authentic s’mores look.

Serve the cupcakes at room temperature for optimal texture and flavor. They pair wonderfully with a glass of milk or a scoop of vanilla ice cream.

Store leftovers in an airtight container at room temperature for up to two days or refrigerate for up to five days. Bring refrigerated cupcakes to room temperature before serving.