1. Place the chopped chocolate in a heatproof bowl set over a pan of simmering water and heat gently over a low heat until melted — do not let the base of the bowl touch the surface of the water.
2. Place the milk in a saucepan and bring gently to the boil. Add the melted chocolate, coffee and chocolate hazelnut spread to the pan and simmer over low heat, whisking all the time, until the chocolate is combined. Remove from the heat and add the hazelnut liqueur to taste, if using. Pour the drink into two cups or heatproof glasses.
3. Whip the cream to stiff peaks and add a spoonful to the top of each drink. Sprinkle with toasted hazelnuts. Serve at once.