Preheat the oven to 170°C (340F/Gas 3). Lay the hazelnuts over the base of a baking sheet. Toast them in the oven for 10 minutes or so until lightly golden. Remove and set aside to cool, then roughly chop (or crush them briefly with the bottom of a rolling pin).
Break up the biscuits by putting them in a bag and bashing them with a rolling pin or by blitzing them quickly in a food processor. It is important they stay in fairly large pieces – you’re not aiming for crumbs.
Melt the chocolate in a small heatproof bowl set over a pan of gently simmering water (or in a microwave). Allow it to cool for a few minutes.
Beat the butter, sugar and salt in a mixing bowl using an electric mixer until smooth and creamy. Whisk in the beaten eggs, a little at a time, to form a smooth batter. Add the cocoa powder and the cooled, melted chocolate. Add the broken biscuits and the dried cherries and stir well to combine. Add the hazelnuts and stir again.
Scoop the mixture out (it will look quite sticky at this stage) onto a rectangle of clingfilm (plastic wrap), aiming for a sort of long oblong shape. Place another piece of clingfilm the same size over the top and wrap the sausage completely. Roll it in your hands to smooth out the shape and then twist the ends. Place it in the fridge to chill.
Once solid and nicely firm, unsheath your sausage and dust in icing sugar, and either wrap it in baking parchment if giving it as a gift or, if you want to go the whole hog, tie it up as you would a proper salami. There are some very instructive videos on YouTube about how to tie salami properly, if you are so inclined. Will keep for up to one week, covered, in the fridge.