1) Preheat the oven to 160 degrees C. Line a 12-hole cupcake or muffin tin with paper liners.
2) Sift together the sugar, flour, cocoa, baking powder, bicarbonate of soda and salt in a large mixing bowl. Combine and whisk the egg, milk, oil, and vanilla in a separate bowl. Using a spatula, combine the dry and wet ingredients, alternating between the two, starting with 1/3 of the dry ingredients and ending with 1/3 of the dry ingredients. Stir until smooth, then add the very hot water and stir until incorporated.
3) Transfer the batter to a pitcher (it is easier to pour the batter than it is to scoop, as it is very thin). Pour the batter into the cupcake tin, filling each cup 3/4-full and bake for 15 minutes. Let the cupcakes cool for 5 minutes before filling.
For the icing:
1) In a large mixing bowl, combine the marshmallow cream, sugar, shortening, salt, and vanilla, and stir until blended.
For the chocolate ganache:
1) Combine the chocolate, butter, hot espresso and golden syrup in a microwave-safe bowl and heat, 20 seconds at a time until the chocolate is melted, stir until smooth.
1) Load the icing into a piping bag fitted with a round pastry tip. Insert the tip into the cupcakes and fill with a little icing, then using the same tip, frost the tops of the cupcakes. Chill the cupcakes in the refrigerator for 15 minutes to set the frosting. Drizzle the frosted cupcakes with tepid ganache.