Preheat the oven to 180ºC (160º fan). Grease and line two 20cm round springform cake tins.
In a large bowl whisk together the melted butter, oil, and water. In another bowl combine the flour, both sugars, cocoa powder, baking powder, bicarbonate of soda, and salt. In a large jug whisk together the eggs, sour cream, and vanilla.
Add the dry mixture to the bowl with the melted butter mixture. Mix together until evenly combined then stir through the wet mixture (you can do this by hand or with a handheld electric mixer). Finally, fold the hazelnuts through and divide the mixture between the cake tins. Smooth to level the mixture and bake for 50-55 minutes, until risen and a skewer inserted into the middle comes out clean. Set aside to cool completely.
To make the frosting, add all the ingredients into a large mixing bowl and beat together using a handheld electric mixer or a freestanding mixer until well combined and smooth, about 5-6 minutes.
To assemble the cake, trim the surface of the cake to level it. Place one cake on the plate or cake stand you want to present it on. Spread about one-quarter of the frosting on the top of the cake and spread to level using a palette knife then top with the other cake. Using the palette knife, spread about half of the frosting around the sides of the cake, smoothing as you go. Spread the remaining frosting over the top, smoothing to level it. Taking a small handful of hazelnuts at a time, press them into the bottom quarter of the frosting to create a rim until a circle around the whole bottom quarter is covered in hazelnuts. Top the cake with the Ferrero Rocher.
