Line 2 baking sheets with nonstick baking paper. Place the egg yolks and half the sugar in a large bowl and beat with a hand-held electric whisk for 5–8 minutes until the mixture is pale and thick and the whisk leaves a trail when lifted above the mixture. Beat in the lemon rind and set aside.
Whisk the egg whites in a clean bowl with a clean hand-held electric whisk until they form soft peaks, then gradually whisk in the remaining sugar and the salt. Fold the egg whites into the egg yolk mixture, then sift in the plain flour and fold in.
Spoon the mixture into a piping bag fitted with a 1 cm (1/2 inch) plain nozzle, then pipe 14–16 x 7 cm (3 inch) lengths on to the prepared baking sheets.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes, or until pale golden. Transfer to wire racks to cool. Dust with icing sugar to serve.