Gooey Chocolate S'mores Cupcakes

Steps

1. 1Start Chocolate Buttercream: Start the buttercream about an hour before you are ready to make your cupcakes. In a small saucepan, over medium heat, melt together butter and chocolate until completely melted. Stir together and then remove from heat.
2. 2Allow to cool on the counter for 10 minutes before placing in the refrigerator until solid, about 1 hour.
3. 3Graham Cracker Layer: Preheat oven to 350° Line cupcake pan with grease-proof liners.
4. 4In your food processor, pulse graham cracker pieces until they are a fine crumb. Add in flour and sugar and pulse until combined. Drizzle in butter and continue to process until completely incorporated. If you don't have a food processor, you can crush the graham crackers with a rolling pin in a plastic bag and mix everything in a bowl.
5. 5Divide graham cracker mixture between cupcakes cups, a little less than one tablespoon per cup, and use your fingers or a shot glass to press mixture down firmly.
6. 6Bake for 5 minutes until tops look set.
7. 7Cupcakes: In a medium microwave-safe bowl, microwave butter until almost completely melted. Add chocolate and oil and microwave for 30 seconds. Stir. Microwave for an additional 15 seconds if necessary, until chocolate is completely melted. Stir until well-combined and set aside to cool.
8. 8In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
9. 9In a large bowl, whisk together sugar, eggs, and vanilla extract. Whisk in cooled chocolate mixture. (It can be warm, but if it is hot to the touch, set aside for another couple of minutes. You don't want to scramble the eggs!)
10. 10Whisk in half of the flour mixture until just combined. Add all of the buttermilk and mix. Whisk in the last of the flour, mixing until just combined.
11. 11Whisk in the coffee (or water). The batter will be thin and runny. It's supposed to look like that.
12. 12Fill cupcake tins just under 3/4 of the way full (if you transfer the batter to a liquid measuring cup first, it makes filling easier). Don't overfill--you might have a tiny bit of batter left over.
13. 13Bake for 15-18 minutes, until a toothpick inserted into the center of the cupcakes comes out clean.
14. 14Take chocolate frosting mixture out of the refrigerator and set on the counter to soften while your cupcakes cool.
15. 15Finish Chocolate Filling and Assemble: Allow cupcakes to cool completely, about half an hour. Once cupcakes are cool, use a handheld electric mixer, or scoop softened chocolate mixture into the bowl of your stand mixer and use the whisk attachment, to whisk mixture on medium-high until light in color, about 2 minutes.
16. 16Using a serrated knife with a sharp tip, cut a cone-shaped piece out of the center of each cupcake. You don't have to be neat about it, and you can discard (or eat) the cone.
17. 17Transfer chocolate frosting to a plastic bag by placing the bag over an empty cup and scooping the mixture inside. Seal bag and snip off one of the corners. Use the bag to "pipe" chocolate mixture into the cupcakes and fill to the top.
18. 18Make marshmallow frosting and top cupcakes with big swirls using a piping bag.
19. 19Turn broiler on high and place cupcakes on a tray in the oven. Watch closely and do not walk away from the oven. Leave cupcakes in just long enough to toast the marshmallow frosting, 2 to 4 minutes.