Line two baking sheets with parchment paper. Prepare a pastry bag fitted with a round 1.5 cm (1/2 inch) tip.

In a medium bowl, beat the egg whites, 50 grams (1/4 cup) of sugar and 1 teaspoon of lemon juice until stiff peaks form.

In another bowl, beat the egg yolks with the remaining sugar, lemon zest, vanilla extract, 1 teaspoon of lemon juice and salt until thick and light yellow.

Sift the flour and potato starch over the egg mixture and gently fold it in with a rubber spatula until smooth and well combined.

Gently fold in the egg whites.

Transfer half of the batter to the prepared piping bag. Pipe the batter into lines about 10 cm (4 inches) long, keeping distance between them.

Repeat with the rest of the batter.

Sprinkle the cookies lightly with icing sugar. Let them rest for about 5 minutes and sprinkle again with icing sugar.

Place the baking sheet in the oven and bake for about 15 minutes until lightly golden.

Let the ladyfingers cool for a few minutes then release them from the parchment paper, with a flat spatula, while they are still warm.

Serve the savoiardi immediately or store in an airtight container up to 2 weeks.