Make the Thumbprint Cookies:
In a medium mixing bowl, whisk together the flour, cocoa powder, and salt. Set aside.
In a large bowl, beat the softened butter and caster sugar on high speed for about 2 minutes, until light and fluffy.
Add the vanilla extract, egg yolks, and melted dark chocolate to the butter mixture. Beat until fully combined.
Gradually add the dry ingredients to the wet mixture, using a spatula to stir until a dough forms.
Use an ice cream scoop or spoon to measure out about 1 ½ tablespoons of dough for each cookie. Roll the dough into balls, then coat them in the finely chopped hazelnuts.
Place the dough balls on a tray and refrigerate for 2 hours to chill.
Preheat the oven to 160°C (320°F) for fan-forced ovens or 170°C (340°F) for no-fan ovens. Line two baking trays with parchment paper.
Spread the cookie dough balls evenly across the trays, leaving space between each. Bake one tray at a time on the middle rack for about 8 minutes. Let the cookies cool completely on the tray.
Shape and Assemble the Cookies:
After baking, use the back of a measuring tablespoon to gently press an indentation into the center of each cookie.
If needed, use a slightly larger round cookie cutter to gently reshape the cookies into perfect rounds by shuffling it around the edges.
Add the chocolate hazelnut spread to the center of each cookie, either by using spoons or a piping bag for a more precise finish.
Allow the cookies to set completely before serving.