You need Marshmellow fluff for this recipe. I hadn't heard of it before- it's an American ingredient. Seems to be basically melted marshmellows. There are various recipes on the back for things like ice cream topping. Also it's apparently nice spread on toast. I found it at my local Sainsburys without much trouble.
For the Sponge
1 large egg
150g (5.5ozs) Caster Sugar
125g (4.5ozs) Plain yoghurt
25ml (1fl oz) whole milk
¼ tsp vanilla essence ( I used extract)
75g (2.5oz) melted unsalted butter
200g (7oz) Plain Flour
80g (3oz) Cocoa Powder
¾ tsp bicarbonate of soda
¼ tsp baking powder
170g (6oz) unsalted butter
280g (10oz) icing sugar
220g (8oz) vanilla marshmallow fluff
Using a mixer cream the egg and sugar until fluffy. Pour the milk, yoghurt and vanilla essence into a bowl and mix together. Then add this mixture in with the beaten egg an sugar. Finally add in the melted butter and mix in thoroughly.
Sift together the remaining dry ingredients, and then to the creamed mixture and mix well on a medium speed.
Now place the batter mix in the fridge to rest for 20-30 minutes.
Pre-heat the oven to 170C and cover two baking trays with baking parchment.
Once rested, spoon or pipe onto the baking trays making 8-10 mounds per tray. These need to be 2-3 cm spaced apart and around 3 -5 cm in diameter. You can make them whatever size you want but remember that they are VERY filling so small is probably better
Bake for 10-13 minutes or until they spring back when touched.
Remove from the trays and place on a wire rack. They need to be completely cool before adding any filling to them.
To make the filling mix the butter and icing sugar together using a hand whisk or standing mixer. Once this is blended add the marshmallow fluff and then mix on a high speed until the mixture is light and fluffy. Place in the fridge for 30 minutes as this will firm up the mixture slightly.
To sandwich the pies, take one sponge and spread a tablespoon of filling on one flat side. Follow this with sticking another sponge, flat side down onto the filling to make a sandwich. Continue this process to make the other whoopee pies.