Place the sugar in the bowl of an electric stand mixer. Zest the yuzus directly onto the sugar, until there is about three teaspoons of zest.
Place the butter, sugar, and salt in the bowl of a stand mixer with the paddle attachment and mix on medium speed until the mixture is smooth, about 2 minutes.
Stop the mixer, scrape down the bowl, add the egg yolk and yuzu juice and mix in on medium speed just until the ingredients are incorporated.
Add all the flour and mix it in on low speed just until all the flour is mixed in. The dough will be crumbly at this point. Increase the mixer speed to medium and mix until the dough has completely come together and forms a dough.
Turn the cookie dough out onto a lightly floured surface. You can also turn it out onto a large piece of baking paper. Use your hands to shape the dough into a rectangle and lightly flour the top of the dough. Do not use too much flour, just enough to keep the rolling pin from sticking.
Roll the dough out to ½ cm thick. Use a 5cm round cookie cutter to cut out the cookies. Place the cookies on baking paper lined baking tray, a couple of centimeters apart.
Chill the dough in the refrigerator for at least 20 minutes. Chilling the dough will help the cookies hold their shape when baking.
Preheat the oven to 150°C.
Bake the cookies for 20-22 minutes. The bottom of the cookies will be lightly golden brown.
Take the cookies out of the oven and allow them to cool for about 5 minutes on the cookie sheet before moving them to a wire rack to completely cool.
If you want to decorate with the yuzu icing, sift the icing sugar into a medium-sized mixing bowl. Add the yuzu juice and stir together. Add a little more if you need until the desired consistency is achieved.
Use an offset spatula or the back of a small spoon to spread the yuzu icing across the top of the cookie. Lay the cookies on a flat surface and let the glaze dry.
Store the cookies in an airtight container for up to two weeks.
