Heat the oven to 180C/160C Fan and line three 20cm/8inch cake tins with baking parchment – leave to the side.
In a stand mixer, beat together the butter and light brown sugar until light and fluffy.
Add in the flour, cocoa powder, eggs, baking powder and beat again briefly till combined. Try not to over beat the mixture!
Divide the mixture between the three tins and smooth it over – bake for 25-30 minutes until a skewer comes out clean when poked, and when the cake springs back.
Once baked, leave the cake to cool in the tin for 10 minutes, and then remove and leave to cool fully on a wire rack. If the cake has domed slightly, leave the cake to cool upside down to flatten it slightly.
For the Buttercream
Beat the Butter on its own in a stand mixer or bowl for a couple of minutes so its really supple and soft.
Add in the Nutella and beat again briefly so its basically combined.
Add in 1/2 of the Icing Sugar and Beat till its incorporated fully, and then add in the second half and repeat.
If the mixture is really stiff, add in 1tbsp of boiling water at a time and beat in before adding enough to soften it slightly.
For the Decoration
For each layer of the Nutella Cake, add a sponge, and then either pipe on slather on some of the Nutella Buttercream. Drizzle over some Nutella.