Preheat your oven to 170°c and line your muffin tray with 6 cupcake cases.
Sift the flour, sugar & baking powder into a large bowl. Add the butter and mix on a medium speed until the mixture resembles a sandy consistency.
Gradually add half the milk to the mixture. Whisk the egg & vanilla extract into the remaining milk and again gradually add to the other ingredients. Mix until only combined - do not over mix!
Using an ice cream scoop, fill your cupcake cases 2/3 full and bake for around 20 minutes until a skewer comes out the cupcakes clean.
When cool enough to touch, transfer to a wire rack to cool completely.
For the Icing
Sift the icing sugar into a large bowl, add the butter and mix on a slow medium speed with a clean tea towel over your mixer to prevent your kitchen filling with icing sugar!
When the butter & icing sugar start to come together, add the marshmallow fluff a spoonful at a time. I find 2/3 of a jar to be just about right - enough marshmallow without it becoming too soft to hold it's own.
Pipe your marshmallow onto your cupcakes best you can - this icing is a very sticky one! Use a spoon to smooth over into a dome-like finish. Running your spoon under hot water throughout this process makes it a bit easier.
Once topped with the marshmallow icing, pop the cupcakes in the fridge for around half an hour whilst melting your chocolate either over a bain marie or in shorts bursts in a microwave.
To finish your cupcakes, remove them from the fridge, hold them upside down and dip the marshmallow icing into your melted chocolate, allowing any excess to drip off before returning them to the fridge to set.
It used to be the case that Marshmallow Fluff was hard to find in the UK but more recently I've been noticing it popping up in most large supermarkets beside the jams, marmalades & chocolate spreads.