Preheat oven to 350 degrees. Line cupcake pans with 24 cupcake liners.
Process the graham crackers in a food processor or crush them in a heavy duty Ziploc bag with a mallet or rolling pin until they are fine crumbs. In a small bowl, mix the graham cracker crumbs with the sugar and melted butter, until completely combined. Add 1 tablespoon of the graham cracker base mixture into each cupcake liner and press into the bottom of the liner with the back of a spoon or the bottom of a small juice cup.
Prepare the chocolate cupcake batter by first sifting together the flour, cocoa, baking powder, baking soda and salt.
In the bowl of a stand mixer or using a large bowl and hand mixer, mix the sugar and oil together on medium-high speed for 3 minutes until light. Add the sour cream and vanilla and mix well, scraping down the sides of the bowl. Add the eggs, one at a time, beating well after each addition.
Add the dry ingredients alternately with the boiling water, mixing just until combined and scraping down the sides of the bowl as needed.
Divide the batter evenly among the cupcake liners, filling each a little over half full. If you have additional batter leftover, you can make some plain chocolate cupcakes, but don’t fill the liners too full or the cupcakes will rise over the edges of the cupcake liners. Press 1 piece of broken up Hershey’s bar down into the chocolate batter of each cupcake.
Bake for 20-24 minutes or until a toothpick inserted into the center of a cupcake comes out clean. Or press on the top of a cupcake to see if it bounces back (my preferred method of testing cupcakes for doneness). Remove from oven and cool completely.
When ready to frost and decorate cupcakes, combine the butter and cream cheese in a large bowl and beat well until smooth. Add the powdered sugar, vanilla, and graham cracker crumbs and mix to combine. Add the cream or milk a tablespoon at a time until the frosting reaches a buttercream consistency, then transfer to a piping bag with a large tip and pipe onto the cooled cupcakes.
Turn your oven to broil. Arrange 24 large marshmallows in a single layer on a baking sheet, then place in the oven under the broiler until toasted to your desired level of toastiness. You can even rotate them to brown all sides of the marshmallow if you want.
Top each cupcake with a toasted marshmallow, a square of Hershey’s chocolate, and a piece of graham cracker to decorate.
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