Preheat a fan-forced oven to 180C (356F) or 160C (320F) for a fan forced oven. Line a cupcake tin with cupcake liners.
In the bowl of a stand mixer fitted with the paddle attachment, add the cocoa powder, plain flour, salt, bicarb soda and sugar. Turn mixer on low speed and allow it to mix for a couple minutes to help everything combine well (alternatively you may do this by sifting the ingredients together). Add the softened butter there is no butter visible and it's well incorporated into the dry.
Next, add milk and eggs in a jug and whisk until well combined.
Add wet ingredients to dry ingredients in a slow and steady stream until no dry ingredients are visible. Scrape down the bowl and mix for another 20 seconds.
Fill each paper 3/4 of the way (using an ice cream scoop to transfer the batter into the cupcake liners makes this a quick and easy process otherwise you can use two tablespoons).
Insert one Ferrero Rocher in each cupcake liner. Bake for 20-25 min or until a toothpick inserted comes out clean. Allow them to cool completely on a wire cooling rack before frosting.
Place your Nutella into a piping bag or zip lock bag and pipe a little swirl on a baking tray lined with baking paper. Place a toasted hazelnut in the centre and then continue piping a swirl on top. It helps if the Nutella isn't warm. If it's a warm day, place the Nutella in the fridge for an hour. Make the swirls about 3 centimeters in diameter. Freeze.
To assemble your cupcakes start off by spraying the cakes with gold mist spray. Place a frozen Nutella centre on top of the baked Ferrero Rocher.
Fit the end of a piping bag with a Wilton 6B piping tip and frost a swirl around the Nutella centre. Sprinkle crushed hazelnuts on top and place some gold leaf on top. Using a pair of tweezers makes this easier. If you can find an easier way let me know!