Nutella cake that is addictive!

Add 1/2 of the powdered sugar and beat until completely incorporated and then add in the second half and repeat.
If the mixture is really stiff, add boiling water 1 tablespoon at a time, whisking before adding enough to soften slightly.

For decoration:

For each layer of Nutella cake, add a sponge and then pipe on some of the Nutella buttercream. Drizzle some Nutella on top.
Repeat each layer until you reach the top.
Decorate with chopped hazelnuts and sprinkles!