Preheat the oven to 350°F (175°C). Line a cupcake pan with liners or grease it with butter and flour.

Sift the flour, cocoa powder, baking soda, baking powder, and salt together in a large mixing bowl.

In a separate bowl, combine the oil, coffee, vinegar, vanilla, and sugar, stirring until well mixed.

Pour the dry ingredients into the wet ingredients and whisk briefly, just until incorporated.

Distribute the batter evenly into the prepared cupcake liners. Bake for 15–20 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and let the cupcakes cool completely.

Finely chop the milk chocolate and place it in a heatproof bowl.

Heat the heavy cream over medium heat until it is almost boiling, or microwave it in 15-second intervals until very hot. As soon as bubbles begin to appear, remove from heat.

Pour the hot cream over the chopped chocolate and let it sit for a few seconds.

Whisk the ganache until the chocolate is fully melted, creating a smooth and luscious mixture.

Allow the ganache to cool to room temperature, which may take 1–2 hours. The ganache must be completely cooled before using it to fill the cupcakes or adding it to the buttercream.

Set aside 170 grams (3/4 cup, 7 oz) of the ganache for the frosting. Use the remaining ganache to fill the cupcakes.

Place the butter in the bowl of a stand mixer and cream on medium-high speed for about 1 minute, until smooth and creamy.

Add the reserved ganache and mix until fully combined.

Turn off the mixer and add the sifted powdered sugar, cocoa powder, and vanilla extract.

Mix on low speed until incorporated, then whip on high speed for about 1 minute until light and fluffy.

If the buttercream is too runny, add more powdered sugar to reach the desired consistency.

In a heatproof bowl, combine the sugar, egg whites, cream of tartar, and salt.

Place the bowl over a pot of barely simmering water, ensuring the bottom of the bowl does not touch the water.

Whisk the mixture continuously while it sits over the double boiler.

Continue whisking until the mixture reaches 140°F (60°C) to ensure the egg whites are safe to consume.

Remove the bowl from the heat and immediately beat the mixture with an electric mixer on high speed for 3–5 minutes, until stiff peaks form.

Add the vanilla extract and mix to combine.

Use a small teaspoon or cupcake corer to remove the center of each cupcake.

Fill the hollowed center with the milk chocolate ganache.

Transfer the milk chocolate buttercream to a piping bag and pipe a mound of frosting onto each cupcake.

Process the graham crackers in a food processor to create fine crumbs. Sprinkle the crumbs over the sides of the buttercream frosting.

Refrigerate the cupcakes while you prepare the marshmallow frosting.

It’s best to prepare the marshmallow frosting just before using it, as it is freshest when made and piped immediately. If left in a bowl or piping bag for too long, the frosting may begin to deflate and lose its shape.

Transfer the marshmallow frosting to a piping bag fitted with your desired tip (a large open star tip works well). Pipe the marshmallow frosting onto each cupcake.

If desired, use a kitchen torch to lightly toast the marshmallow frosting.

Store the S'mores Cupcakes in the refrigerator for 4–5 days.