Preheat the oven to 190°C / fan 170°C / gas 5. Butter a 12 hole muffin tin.
Cream together the butter and sugar with an electric whisk.
Add the eggs and whisk until they are combined.
Add the self-raising flour and baking powder and whisk until everything is combined.
Add the vanilla extract and give it one final whisk.
Divide the mixture into the cases and cook for 15/20 minutes.
Once your cakes are cooked let them cool on a wire rack for 5 minutes.
With a sharp pointy knife cut a hole in the middle of the cake but not going all the way through to the bottom. Place the marshmallow in the hole and repeat on each cake. You don't need to keep the middles of the cakes but you might want to try some along the way!
Now make your frosting. Cream together the butter and icing sugar on a low setting and gradually turn the speed up until you have a thick paste.
Add the vanilla extract and milk and with a spatular give it a good mix.
Put the frosting into a piping bag with a nozzle of your choice and ice each cake.
Grate dark chocolate all over your cakes and decorate with mini eggs.