Preheat your oven to 350°F (175°C) and position the oven rack in the middle.

Line two cookie sheets with baking paper.

Whip the egg yolks with half of the granulated sugar until pale and fluffy, then add vanilla extract and lemon zest if using.

Whisk egg whites until soft peaks form, then gradually add remaining sugar and whisk until stiff peaks hold.

Fold whipped egg whites into the yolk mixture carefully to retain volume.

Whisk together flour and cornstarch, then fold into the wet mixture without over-mixing.

Pipe 3-inch long cookies onto the lined baking sheets, spaced an inch apart.

Dust piped cookies lightly with powdered sugar and bake for 12-15 minutes until golden.

Let cookies cool on the tray for a few minutes before transferring to a wire rack.