Line a 24 x 8cm loaf tin with plastic wrap or baking paper, leaving an overhang that will enable you to remove the ice cream. Toast 40g (1½ oz) of the hazels in a dry frying pan to pale gold, then chop them to small pieces by hand. Keep aside.
Heat the milk over a gentle heat in a large heavy-bottomed saucepan, then add the chocolate, stirring to melt. Add the cocoa, whisking all well to smooth & remove from the heat.
In a medium bowl, whisk the eggs with the sugar, vanilla & a pinch of salt until pale & creamy. Slowly dribble in some hot chocolate mix, whisking so it doesn't scramble. When all the chocolate is incorporated, scrape the mixture back into the pan, add the chopped hazels to infuse over a very low heat. Whisk continuously for a couple of minutes just to cook the eggs through. There is no need to thicken. Remove from the heat, whisking now & then as it cools. Whisk in the cream, then cool in the fridge until well chilled.
Strain the mixture directly into your prepared tin (you no longer need the chopped hazels – so you can snack or discard). Put in the freezer to set.
Halve some of the remaining hazelnuts & quarter the rest. Toast them all in a dry frying pan with a light sprinkling of salt, then leave to cool. To serve the ice cream – lift it out from the tin & cut into chunky slices. If it is too frozen, wait a little. Serve each with a good dollop of whipped cream & some toasted hazelnuts.