Using a stand mixer fitted with the paddle attachment, beat the butter, lemon zest, and fleur de sel on low speed until smooth (about 90 seconds).

In a separate bowl, whisk the egg yolks while gradually adding the sugar. Continue whisking until the mixture is light and fluffy (about 1 minute).

With the mixer on low, gradually add the yolk mixture to the butter. Stop to scrape down the sides of the bowl to ensure everything is fully combined.

Sift together the flour and baking powder. Add to the butter mixture and mix by hand with a wooden spoon until fully incorporated. Do not overmix.

Shape the dough into a 1-inch (3 cm) thick rectangle. Wrap in plastic wrap and refrigerate for at least 1 hour. Dough can be made up to 5 days in advance.

Line two baking sheets with parchment paper. Cut the dough in half. Roll one piece between two sheets of parchment paper to about 1/3–1/2 inch (approx. 1.25 cm) thick. Remove the top sheet.

Using a 2-inch (5 cm) round cookie cutter, cut out circles. Use a pastry scraper or spatula to transfer the biscuits to the prepared trays, spacing them 2 cm apart. Repeat with second half of dough. Chill trays in the fridge or freezer until biscuits are firm.

Preheat oven to 180°C fan ( 200°C conventional). Position a rack in the centre of the oven.

In a small bowl, beat the whole egg with 1 tsp of water. Remove one tray of biscuits from the fridge/freezer and brush the tops with egg wash. Bake for 12–15 minutes or until edges are golden.