To toast the hazelnuts, lay them in a single layer on a baking sheet and toast in a 300 degree F oven for about 30 minutes (time may vary), shaking the pan every so often to rotate them. As soon as they start to turn golden brown and you can smell them, remove them from the oven (they can go from perfect to burnt very quickly) and swirl pan around to try and loosen the skins even more. When cool enough to handle, you can rub a handful in between both your hands to further remove the skins. Its really hard to get all of it off, but do your best and give up when it looks like anymore won't fall off.
Preheat oven to 350 degrees F. Line two baking sheets with a silpat or parchment paper.
In a large food processor, process toasted hazelnuts, flour, and salt until finely ground, 45 seconds to a minute.
In a stand mixer with the paddle attached, cream the butter and sugar together until very light and fluffy on high speed, 5-7 minutes. Scrape the sides, then add the egg yolks and vanilla and beat on low speed until fully mixed. With the mixer running on low speed, spoon in dry mixture one spoonful at a time until all of the dry ingredients have been added, then stop mixing as soon as all the ingredients have incorporated.
Use a tablespoon to scoop up an even spoonful of dough and place on your baking sheet. Roll each mound of dough into a ball. Use the back of a 1/4 teaspoon to press a well into each ball. I prefer my wells to be deeper than wide, but be sure to not go all the way to the bottom. Spread the cookies out so that you have at least an inch in between cookies. They don't spread much while cooking, but they do a bit.
Bake in preheated oven for 10 minutes. Remove from oven and while still hot, use that 1/4 teaspoon to reshape your wells. fill each well with 1/2 teaspoon of lemon curd.* Return cookies to oven but rotate the baking sheet from its original position to ensure even cooking. Bake until cookies are just turning golden brown around the edges, another 8-10 minutes.
Allow cookies to cool on the baking sheet for 5 minutes and then cool completely, another 45 minutes, on a wire rack. Dust with powdered sugar, if desired.
Notes
* You can use only lemon curd, or you can do what I did and add in other flavors of preserves with the lemon curd.