Beat the egg white with the salt until stiff. Gradually add the sugar and continue beating until the egg white turns glossy. Add the almond extract, lemon zest and almonds, carefully fold in using a rubber spatula.
Transfer the mixture to a piping bag with a smooth nozzle (approx. 4 mm in diameter). Pipe hazelnut-sized mounds onto two baking trays lined with baking paper, ensuring they are well spaced out.
Approx. 15 mins. in an oven preheated to 150 °C (convection). Remove from the oven, leave to cool on a cooling rack.
Dust the amaretti biscuits with icing sugar.