For the curd, rub the zest into 100 g sugar. Mix in the yolks and salt until slightly pale. Use a nonreactive bowl and silicone or wooden utensils.

Heat 60 g lemon juice just to boiling. Slowly stir it into the yolk mixture, then return everything to a nonreactive saucepan.

Stir constantly over low heat for about 4–5 minutes, until the mixture thickly coats a spatula. Do not let it boil. Remove from the heat, stir in the butter and strain if desired.

Cover the curd directly on its surface and cool completely.

For the dough, rub the remaining zest into 150 g sugar. Whisk in melted butter, eggs, 30 g lemon juice and vanilla. Mix in the flour, xanthan gum, baking powder and salt. The dough will be very soft; do not add extra flour.

Refrigerate for at least 2 hours or overnight.

Heat a conventional oven to 350°F / 180°C. Line two baking trays. Scoop 2-tbsp portions of dough into powdered sugar and coat well before rolling into balls.

Arrange with plenty of space, about 8 per tray. Press a well halfway into each ball with a 1/2-tbsp measuring spoon. Add a generous teaspoon of cooled curd.

Bake one tray at a time for 9–12 minutes, until the edges have spread and cracked and the curd is slightly puffed but not bubbling. Keep the waiting tray refrigerated.

Cool on the tray for 5–10 minutes, then move to a rack.