First make your cupcakes. Preheat the oven to 180c/350f. Take a 12-hole metal muffin tin and spray with a little cooking oil (alternatively you could wipe with a little butter, or line with muffin cases).
Break up the dark chocolate and place in a small bowl with the coconut oil. Place in the microwave and melt by putting in the microwave in 30 second blasts, then stirring in between each blast. Put to one side to cool a little.
In a large bowl whisk the sugar and egg until combined, then stir in the vanilla and yogurt. Next add the coconut flour, GF plain flour and baking soda. Mix until just combined. Finally pour in the melted chocolate and coconut oil. Mix to combine and spoon out into the muffin tray. You want to fill the muffin holes until the mix is just under the rim. The cakes will rise, but not so much that they'll spread all over the tin. You may find you have a little left over, just cook this in a separate cake case or ramequin (cooks treat). Place in the oven and cook for 14-16 minutes - until a skewer inserted into the centre of a cake comes out clean. Once cooked, take out of the oven, leave to cool for a couple of minutes, then remove from the tray and allow to cool completely on a cooling rack.
Next make the marshmallow. Place the 180g/6.3oz sugar and water in a small heavy-bottom pan. Place a candy thermometer in the pan and heat on a medium heat until the temperate reaches 240f (soft ball stage). Don't mix at any point or it will crystallize.
Whilst the sugar and water are heating, place your egg whites in a bowl with the vanilla and salt and whisk (using an electric whisk on medium) until you get soft peaks (i.e when you lift the whisk, the mixture very slowly sinks back in on itself).
Staying on the medium setting, slowly add the 30g/1oz caster sugar until incorporated.
When your syrup reaches 240f, take it off the heat, and whisking the whole time,very carefully pour HALF of it (You can discard the other half, I find it's too difficult to make a smaller portion of the syrup) into the egg mixture in a very thin stream at the edge of the bowl (don't pour it on the whisk itself as you may end up being splattered with boiling hot syrup). Continue to whisk until you reach the stiff peaks stage (meaning the mixture will hold it's shape when you life the whisk out).
For the ganache, place the cream into a small pan and heat on medium until it's almost boiling (you'll see a couple of bubbles rise to the top when it's time to turn it off). Turn off the heat and add in the chocolate. Cover the pan and leave for 5 minutes. After 5 minutes, take of the lid and whisk the chocolate and cream. It should turn to a glossy sauce. Leave the sauce to thicken (about 10-15 minutes).
Now it's time to assemble the cakes. Place the marshmallow fluff into a piping back fitted with a large, plain round nozzle. Starting from the outside and working towards the centre, pipe the marshmallow on top. Repeat with the remaining cupcakes.
Give the Ganache a quick whisk and then drizzle on top of the marshmallow.
Any leftover marshmallow can be covered and refrigerated for up to a day (after this time the syrup starts to separate back out again).The cupcakes are best eaten on the same day, but they will last a further day or two covered and refrigerated..Nutritional Information is per cupcake.