Place the chestnuts on a baking tray lined with baking paper.
Approx. 10 mins. in the centre of an oven preheated to 200°C. Remove, leave the chestnuts to cool on the tray. Blitz the almonds and chestnuts in a food processor, transfer to a bowl.
Beat the egg white with the salt until stiff. Gradually add the sugar, continue beating until glossy and stiff.
Using a rubber spatula, carefully fold in the chestnut & almond mixture and lemon zest.
To shape the amaretti biscuits
Using a tablespoon, create walnut-sized mounds on a baking tray lined with baking paper, leaving plenty of room between each biscuit.
Approx. 20 mins. in the centre of an oven preheated to 150 °C (convection). Remove from the oven, allow the amaretti biscuits to cool on a rack.