Making the Cookies

1. Preheat the Oven:
   • Begin by preheating your oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. Prepare the Hazelnuts:
   • Toast the hazelnuts in a dry skillet over medium heat until they are fragrant and slightly golden. Allow them to cool before grinding them finely in a food processor.
3. Mix the Dry Ingredients:
   • In a medium bowl, combine the ground hazelnuts, all-purpose flour, and salt. Set aside.
4. Cream the Butter and Sugar:
   • In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy. This should take about 3 minutes.
5. Add the Egg Yolk and Vanilla:
   • Mix in the egg yolk and vanilla extract until fully incorporated.
6. Combine Wet and Dry Ingredients:
   • Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
7. Shape the Cookies:
   • Roll tablespoons of dough into balls and place them on the prepared baking sheets. Use your thumb or the back of a spoon to create an indentation in the center of each cookie.
8. Bake:
   • Bake the cookies in the preheated oven for 10-12 minutes, or until the edges are lightly golden. Remove from the oven and allow them to cool on the baking sheets for a few minutes before transferring to a wire rack.

Making the Chocolate Ganache

9. Heat the Cream:
   • In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Do not let it boil.
10. Combine with Chocolate:
    • Place the finely chopped dark chocolate in a heatproof bowl. Pour the hot cream over the chocolate and let it sit for a minute to melt the chocolate.
11. Stir Until Smooth:
    • Add the tablespoon of butter and stir the mixture until it is smooth and glossy.
12. Fill the Cookies:
    • Once the ganache has cooled slightly, spoon it into the center of each cookie. Allow the ganache to set before serving.