Cupcakes:Preheat oven to 350 degrees F and line a muffin pan with paper liners. Set aside.
In a large bowl, place chocolate and cocoa powder. Pour hot coffee over and let sit for 1-2 minutes. Whisk until smooth. Set aside for 10-15 minutes or until cooled to room temperature (the amount of time this takes will depend on the temperature of your liquid -- mine wasn't very long at all, just warm enough to melt the chocolate).
Add canola oil and eggs and whisk until smooth. Add the vinegar, vanilla, flour, sugars, salt and baking soda and whisk until smooth.
Fill paper liners ¾ full -- I got 14 cupcakes. Bake for 15-18 minutes, or until a toothpick comes out clean. Set aside to cool.
Marshmallow filling:Place about 1" of water into a small pot and bring to a simmer on the stove. Place a larger glass bowl over top, so that it's not touching the water. Add the egg white, sugar and cream of tartar and whisk until combined.
Continue heating and whisking until sugar is dissolved and egg mixture is warm but not hot.
Beat with an electric mixer until cooled to room temperature and soft peaks form -- this can take some time, just be patient. It may take 5-8 minutes.
Frosting:With an electric mixer, beat butter and Nutella together until combined. Add powdered sugar and beat on low until combined. Add heavy cream and beat on high until light and fluffy, adding more sugar or cream to reach desired consistency if necessary.
Assembly:Using a paring knife, hollow out the middle of the cupcakes, making sure not to cut through the bottom. Don't discard the cake -- those are great for snacking!
Fill cupcakes with marshmallow filling, then frost with Nutella frosting and garnish with graham cracker crumbs and marshmallow bits as desired. Store in the refrigerator for up to 4-5 days.