In a dry frying pan, toast the hazelnuts over a medium heat for 5 minutes until lightly browned, then whizz in a food processor for 4-5 minutes to a smooth paste.
Meanwhile, melt the chocolate in a heatproof bowl over a pan of simmering water until smooth and glossy.
Add the melted chocolate to the food processor with the hazelnuts, along with the icing sugar and cocoa powder. Whizz until smooth, then transfer to a jar. Chilled, this spread will keep well for up to 1 month.