The Alberti brothers serve this Nutella cake with an espresso martini.
Preheat the oven to 350ºF (180ºC/gas mark 4).
In a large bowl, whisk the egg and sugar together until the sugar has dissolved and the mixture is pale in color. Then add the Nutella and mix well until fully combined.
In a separate bowl, add the ground hazelnuts, cocoa powder, flour and baking powder. Then add the dry mixture to the bowl with the eggs and Nutella and mix well until fully combined.
Pour into a 9-inch round round greased and lined spring-form loose-bottomed cake tin and cook for 40 minutes or until the cake begins to come away at the sides; then remove from oven and let cool on a rack.
In a separate bowl add the mascarpone, Nutella and icing sugar and whisk together until combined into a smooth paste.
Take the cooled cake out of the tin carefully and, using a spatula, spread the Nutella and mascarpone mix generously all over the cake, covering the top and sides. Then sprinkle the coarsely chopped hazelnuts over the cake.