Preheat the oven to 350 degrees F. Line a cupcake tin with liners.
Set up a double boiler by filling a small saucepan with an inch or two of water.
Place a heatproof bowl that is bitter than the saucepan on top.
Heat the saucepan on medium-high until the water starts to boil, then turn off the heat.
Add the butter, oil, coffee, cocoa, chocolate, and sugar together.
Stir well and keep mixing until the butter and chocolate are melted and well-combined. Remove the bowl away from the saucepan.
Add in the vanilla, salt, and eggs and beat vigorously.
Stir in the flour and baking soda. Mix until just combined.
Evenly divide the batter among the cupcake liners and bake for 16 - 18 minutes, or until a toothpick comes out with a few crumbs attached.
Allow to cool completely. While the cupcakes are cooling, make the frosting.
Cream the butter and then add the powdered sugar and cocoa one cup at a time. Sprinkle in the salt and add in the food coloring.
Mix in the heavy cream a tablespoon at a time until you reach the desired consistency. The frosting may not look completely black yet, but the color develops over time.
Fill a piping bag with the frosting and pipe on top of the cupcakes.
Heat one marshmallow at a time for about 10 seconds. Make strings with the marshmallow by pinching some and stretching it over the frosting. If the marshmallow gets too stiff for you to stretch, pop in the microwave for another 5 seconds. Repeat using as many marshmallows as required to cover all the cupcakes.
Store the cupcakes in an airtight container for up to three days