Requires: x1 piping bag with Wilton 1A piping tip or of your choosing
Preheat the oven to 200°C conventional (180°C fan-forced/ 392° Fahrenheit/Gas 6). Line a large shallow baking tray with baking paper.
Use an electric mixer (or hand beater) to beat the egg yolks for 5 minutes. Add half of the sugar and continue beating for a further 5 minutes until light and fluffy.
In a separate bowl, beat the egg whites with a pinch of salt until stiff peaks form. With the mixer running, add the remaining sugar and beat until combined.
Use a spatula to delicately fold the stiff egg whites into the egg yolk mixture in 3 lots. Fold the 00 and potato flour delicately using the spatula into the biscuit mixture. Fold in the lemon zest.
Transfer biscuit mixture to a piping bag (use Wilton 1A piping tip or of your choosing). Pipe fingers, about 8cm long and spaced evenly apart on the prepared baking tray. Sprinkle with caster sugar (approximately 1 tablespoon per 2 biscuits), then sift the icing sugar over the biscuits. Wait 2 minutes and then sift icing sugar again over the biscuits.
Bake for 8 minutes or until cooked through (quickly opening and then closing the oven after 5 minutes cooking time to allow air in to dry the biscuits). Allow to cool before serving.