In a mixing bowl fitted with a paddle attachment, combine butter, sugar, and salt.
Once creamed, add egg. Scrape down sides.
Pass hard boiled yolks through a fine mesh sieve, then add to the batter. Add flour last, little by little.
Do no overmix- once flour is combined, turn mixer off and scoop out dough.
Place dough between two pieces of parchment paper and press down to form a disk.
Freeze dough for 1 hour.
Remove dough from freezer, preheat oven to 350 degrees F
Between the parchment, roll dough to ½ “ thickness. Use a 2 ½ “ round cookie cutter to punch out as many cookies as you can. You may roll and reuse dough until there is none left.
Place dough 1” apart from eachother on a cookie tray fitted with parchment.
Sprinkle a little fleur de sel on top of each cookie.
Freeze for 10 minutes on cookie tray, then bake for 10 minutes.Remove from oven, cool to room temp and enjoy!!