01 -
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and confectioners' sugar until light and fluffy, about 3 minutes. Add the egg yolk and vanilla; mix to combine, about 1 minute more. Add the flour and salt; mix just until combined.
02 -
Prepare a piece of parchment paper about 30cm long. Turn the cookie dough out onto the parchment and form it into a rough log along one long side of the paper. Use the paper to help form the dough into a rounded log shape, about 5cm in diameter. Wrap the dough in the parchment paper.
03 -
Transfer the wrapped dough to the refrigerator and chill for 1½ hours until firm.
04 -
Preheat the oven to 180°C, with racks in the upper and lower thirds. Line two baking sheets with parchment paper.
05 -
Use a sharp knife to cut the cookie dough into 8mm-thick slices. Transfer the slices to the prepared baking sheets, staggering them and leaving at least 1.5cm between cookies.
06 -
In a small bowl, whisk the egg white with 1 tablespoon cool water to create an egg wash. Working a few cookies at a time, brush the surface with the egg wash, then place a flower in the center. Gently brush over the flower again with egg wash and sprinkle with superfine sugar.
07 -
Bake the cookies for 13-15 minutes, rotating the sheets from front to back and top to bottom halfway through. The cookies are done when they're evenly golden brown on the bottom (they won't brown much on top).
08 -
Allow the cookies to cool completely on the baking sheets before serving or storing.