1. In a standing mixer with paddle attachment, cream the butter and both sugars until fluffy. Add the egg yolks and keep mixing until just blended.
2. Sift together flour and salt. Add to butter mixture, on low speed, until almost incorporated. Stir in hazelnuts and mix until just combined. Take care not to over mix the dough.
3. Divide dough into four portions. Flatten each portion and wrap in plastic wrap. Chill dough in refrigerator for at least 2 hours, or overnight.
4. Preheat oven to 325°. Take one portion of dough and place it on a piece of parchment paper. Lightly flour the top, and roll out to about ¼-in. thickness.
5. Punch out cookies using a round 1.5-in. cutter. Place cookies on a parchment-lined sheet, and garnish tops with a pinch of fleur de sel. Bake for 10 minutes, rotating sheet after five minutes, until cookies are golden brown around the edges. Repeat with rest of dough. Cool completely and store in an airtight container.
Note: Dough can be frozen.