Start by adding hazelnuts to a food processor. I bought toasted hazelnuts from the market and so I could use them as is. If you have raw hazelnuts, roast in the oven at 350 F degrees for around 8 minutes. Cool to room temperature, then remove the skin by rubbing the hazelnuts between a kitchen towel or your hands.Pulse the hazelnuts a few times in your food processor until finely ground and then set aside.
Now, to the steel bowl of your stand mixer (or use your hand mixer), add the butter and sugar. Beat using the paddle attachment for 2 minutes on medium speed until the sugar and butter are well combined and mixture is creamy.Then add the vanilla and mix until combined.
Add the prepared grounded hazelnuts and all purpose flour. Also add pinch of salt if using unsalted butter. Mix using the paddle attachment until everything comes together as a dough. It will be a soft dough.
Flatten the dough, wrap with a cling sheet and refrigerate for 24 hours. If you don't have time, I recommend at least chilling the dough for 4 hours. This is important, if you don't chill the dough, the cookies will spread in the oven.
Once the dough is chilled (I chilled mine for 24 hours), take it out of the refrigerator. Let it sit for 10 minutes and then give it a whack using a rolling pin and start rolling it. Roll it 3/4 to 1/2 inch thick.
Cut cookies using a cookie cutter. I used a fluted rectangular cookie cutter and got 18 cookies from the dough. Preheat your oven to 325 F degrees. Arrange the cookies on a baking sheet lined with parchment paper with an inch or two gap between each cookie. Bake at 325 F degrees for 20 to 25 minutes. Remove from the oven and then transfer onto a wire rack to cool completely.
These can be enjoyed as such but if you want to you can top them with some chocolate. Melt some melting wafers and then spread the melted chocolate over the cookies using a flat butter knife or spatula. Top with crushed toasted hazelnuts and fleur de sel (optional). Enjoy!