Pour 5 cm (2 inches) of water into a medium saucepan and bring to the boil. Cover with a large heatproof bowl, making sure that the water does not touch the base of the bowl. Reduce the heat so that the water is simmering, then add the egg yolks, sugar, sherry and cinnamon to the bowl. Whisk for 5–8 minutes until very thick and foamy, and the custard leaves a trail when the whisk is lifted above the mixture.
Drain off some of the cherry syrup and then tip the cherries and just a little of the syrup into a small saucepan. Warm through, then spoon into 4 dessert glasses.
Pour the warm zabaglione over the top and decorate with the amaretti biscuits. Serve immediately.