Preheat the Oven: Set your oven to 350°F (175°C) and line baking sheets with parchment paper.
Mix Dry Ingredients: In a bowl, whisk together flour, salt, and chopped hazelnuts.
Cream Butter and Sugar: Beat softened butter and sugar until fluffy.
Add Egg Yolk and Vanilla: Mix in the egg yolk and vanilla extract until smooth.
Form Dough: Gradually combine the dry ingredients with the butter mixture until dough forms.
Shape Cookies: Roll dough into 1-inch balls and place on a baking sheet.
Make Thumbprint: Use your thumb to press a well into each cookie.
Bake Cookies: Bake for 10-12 minutes until golden.
Prepare Ganache: In a saucepan, heat heavy cream and ground coffee until it just simmers.
Make Ganache: Pour hot cream over chocolate chips and stir until smooth. Add vanilla extract.
Fill Cookies: Spoon ganache into the thumbprint wells of each cookie. Let cool and set.