Preheat oven to 325 degrees F. Grease and flour 12 muffin cups.
In a small bowl, thoroughly stir together the flour, baking soda and salt.
In a mixing bowl, whisk together the boiling water, cocoa powder, chocolate chips and espresso powder.
Whisk the sugar, sour cream and oil into the mixing bowl, then whisk in the eggs and vanilla.
Add the flour mixture and stir with a wooden spoon or rubber scraper until batter is smooth.
Divide batter evenly between muffin cups and bake for 18-22 minutes.
Let the muffins cool on a wire rack for 10 minutes, then turn from the pan and let cool completely.
To make the filling, sprinkle the gelatin over the water in a large microwave-safe bowl and let soften for five minutes.
Microwave for about 30 seconds or just until water boils around the edges. Add the butter, vanilla and salt and whisk until smooth. Let cool until just warm to touch, then whisk in the marshmallow crème until smooth. Refrigerate for 30 minutes or until somewhat set -- mine was thick, but not entirely set.
Put 1/3 cup of the marshmallow mixture in a pastry bag fitted with a writing tip. This will be used to make the swirl on top. The rest will be the filling. Alternatively, you can skip setting aside some marshmallow mixture and make a small batch of buttercream frosting (see note for directions).
To fill the cupcakes, insert the tip of a knife at at 45 degree angle about a ¼ inch from the edge of the cupcake edge. Carve out a cone. Fill the carved out part with marshmallow cream. Trim the tip of the cone and use the top flat part to cap the hole.
To make the glaze, combine the chocolate chips and butter in a small bowl and microwave for 30 seconds. Stir until melted and smooth. Let cool for 10 minutes (I didn't let mine cool) and spoon over the cupcakes. Let set and then pipe swirls over the top using your reserved icing or the easy icing recipe in the notes.