I first made this for a birthday, it is absolutely scrumptious. The icing may sound hard but it's really quite easy, though it looks very impressive.
Silicone cake moulds work really well with this recipe as they peel off the cake really easily.
I got the basic chocolate cake recipe for this from "Cookbook for girls" by Denise smart, I highly recommend it for any young girls who love to bake as the recipies are simple but effective.
You could make this a 2 tier cake by dividing the mixture into two 20cm cake tins and using a bit more Nutella icing for the middle.