Position racks in top and bottom thirds of oven, and preheat to 350°F. Line 2 baking sheets with parchment paper.

In bowl, whisk together flour, potato starch and salt.

Place a fine-mesh sieve over a large bowl. Using a rubber spatula, press egg yolks through, scraping the sieve. Add butter and sugar. Using a hand mixer, beat on medium speed until pale and fluffy, 2 to 3 minutes. Beat in vanilla and lemon zest. Stir in flour mixture until a dough forms and no patches of dry flour remain. Form the dough into a ball.

On lightly floured surface, roll out dough to scant ½-in (1-cm) thickness. Using 2½ in. (6½ cm) flower-shaped cookie cutter, cut out shapes, rerolling scraps. Place cookies 1 in (2½ cm) apart on prepared pans.

Using ½ in (1 cm) round piping tip, cut centre out of each cookie.

Bake cookies in top and bottom thirds of oven, rotating and switching pans halfway through, until edges are golden brown, 10 to 15 minutes.

Let cool on pans for 1 minute. While they’re still hot, dust cookies with icing sugar, then transfer to rack to cool completely.