In the bowl of a mixer, beat the butter at medium speed until smooth and creamy. Add the sugars and fleur de sel and continue beating until smooth and velvety.
Reduce the mixer speed and add the two egg yolks, mixing well.
Add the flour and mix until the dough is homogeneous.
Gather the dough into a ball and shape into two 9″ (23 cm) long rolls.
Wrap them in plastic wrap and refrigerate for at least 2 hours.
Preheat the oven to 350 F (180 C).
Line two baking sheets with parchment paper. Set aside.
Beat the egg yolk until smooth.
Brush the rolls of dough with the egg yolk, then sprinkle the surface of the rolls with maple sugar (or lavender sugar).
Cut the rolls into 1/3″ (0.8 cm) thick slices and place them on the prepared baking sheets, leaving about 1″ (2.5 cm) of space between each cookie.
Bake for 17 to 20 minutes, or until the cookies are lightly golden.