Preheat your oven to 180ºC/gas mark 4. Place the flour, sugar, cocoa powder, bicarbonate of soda and a pinch of salt into a mixing bowl and combine. Set aside.
Pour the buttermilk, egg, butter, vinegar and vanilla extract into a separate large mixing bowl and whisk together. Heat the Nutella in 10-second bursts in the microwave until smooth and runny.
Pour the flour mixture into the buttermilk mixture and stir until fully mixed. Mix in the melted Nutella until combined.
Lightly grease doughnut pans for 10 doughnuts and dust with the remaining cocoa powder. Pour the batter into a piping bag and pipe into the doughnut moulds.
Bake the doughnuts for 10-12 minutes or until a toothpick comes out with just a few wet crumbs when inserted into the centre of a doughnut. Remove from the moulds and carefully turn out on to a cooling rack. Allow the doughnuts to cool completely.
To make the glaze, add all the ingredients except the icing sugar to a medium saucepan and heat over a medium heat, stirring until smooth. Gradually add the icing sugar, stirring continuously, then remove from the heat. Transfer to a heatproof bowl and set over a small saucepan of simmering water to keep it from stiffening.
To coat the doughnuts, dip the tops of each doughnut into the warm glaze and shake to remove any excess. Place the doughnuts on a cooling rack, glaze side-up, and quickly sprinkle with the chopped nuts. Let the doughnuts sit for 10 minutes to allow the glaze to harden before serving.