Preheat your oven to 350°F. Line a cupcake pan with 4 cupcake liners.
In a small, microwave-safe bowl, combine butter, chopped chocolate, and vegetable oil. Microwave for 30 seconds, and stir until smooth, microwaving for an additional 15 seconds if necessary. Set aside to cool slightly.
In a small bowl, whisk together all-purpose flour, cocoa powder, baking powder, and salt. Set aside.
In a medium bowl, whisk together granulated sugar, egg yolk, and vanilla. Whisk in slightly cooled chocolate mixture until smooth.
Whisk in flour mixture until just combined. Add the milk and hot coffee and whisk until smooth. The batter will look thin.
Spoon or pour batter into prepared cupcake cups, filling 2/3 of the way full. Bake for 16 to 19 minutes, until a toothpick inserted in the center comes out clean or with only a few dry crumbs.
Cool cupcakes in pan for 5 minutes before transferring to a cooling rack to cool completely, 15 to 20 minutes.
Use a serrated knife to cut a cone-shaped piece out of the center of each cupcake. Cut the base of the cone/center of the cupcake off and discard (or eat!) and save the cupcake top.
Spray two spoons with cooking spray and set next to your hollowed out cupcakes. In a medium, microwave-safe bowl, combine marshmallows and corn syrup. Microwave for 20 to 30 seconds until marshmallows are bloated and melty. Stir until smooth. Mixture will be sticky! Use greased spoons to fill hollows of cupcakes and place the cupcake tops back on.
Once your cupcakes are filled, in a small saucepan, combine milk, butter, corn syrup, and chocolate. Heat over medium, stirring occasionally, until melted and combined. Turn heat off, but leave the pot on the burner and whisk in sifted powdered sugar until glaze is smooth.
Transfer glaze into a small bowl wide enough for dipping and dip each cupcake down to the paper wrapper. Allow glaze to set for 5 minutes.
In a small bowl, whisk together powdered sugar and 1/2 teaspoon of milk. Add milk by the 1/4 teaspoon until icing is smooth and when you run a knife down the center of the bowl it takes 20 seconds for the icing to completely fill back in. (You can add more powdered sugar if necessary to reach the right consistency.)
Transfer icing to a small plastic bag and snip off a very small piece of the tip. Practice writing a line or two on a plate before adding swirls to the top of the cupcakes. Let sit for 5 minutes and enjoy!
