Butter and flour a 23 cm (9 inch) round cake pan with a hole in the middle.

Preheat oven to 160° C. Put the hazelnuts on a baking tray. Bake for 6 minutes until nicely roasted. Fold them in a kitchen towel and rub to get rid of as much of their skin as possible. Don’t worry if you have some skin left. Roughly chop in a food processor.

Increase oven temperature to 175° C. Mix the cocoa with the boiling water and stir until smooth. Mix in the Nutella. Leave to cool slightly.

In a bowl, sift the flour with the baking powder, baking soda and salt.

Put the butter and sugar in a mixing bowl of a stand mixer. Beat with the flat beater until creamy and fluffy (about 2 minutes). Add the eggs, one at a time, beating well after each addition. Add the vanilla essence and beat until combined.

Add the sour cream and the Nutella-cocoa-water mixture. Beat until smooth. Lower the speed of your mixer and add the flour. Mix on low speed until batter is smooth. Don’t over beat.

Put batter into prepared pan. Bake for 45-50 minutes, until a cake tester comes out clean.

Make syrup while cake bakes. Heat Nutella and milk in a small saucepan over medium heat, stirring until smooth.

Take cake out of the oven and leave to cool in the pan for 5 minutes. Invert cake on a serving platter. With a pastry brush, brush all over with the Nutella syrup. Leave cake to cool for 30 minutes.

Make ganache while cake cools. Heat Nutella and cream in a small saucepan over medium heat, stirring until smooth. Leave ganache to cool until slightly thickened.

Pour ganache over cake. Sprinkle with chopped hazelnuts. Cool completely and serve.