Preheat oven to 325F/160C. Line 2 baking trays with baking paper
Chop hazelnuts in a food processor. Do not over chop. Reserve 1/4 cup of hazelnuts to sprinkle on top of the cookies.
Place softened butter, sugar, vanilla in a stand mixer and beat until pale and fluffy. Can use a handmixer if you don't have a standmixer.
Add egg yolk and mix again. Make sure to scrape the sides of the bowl, so everything is mixed together.
Sift flour and salt into cookie mixture. Mix on low until just combined. Add 3/4 cup hazelnuts and mix until just combined.
Roll the dough into 12 balls and place onto prepared trays. Using a fork, press down on the cookie a couple of times to flatten it out. Sprinkle chopped hazelnuts on the tops, you may need to press the nuts gently into the cookie.
Bake cookies for about 15-20 minutes. Check at the 15 minutes stage. The cookies are cooked when the edges are slightly browned on the edges.