Step 1: Toast and Grind the Hazelnuts

1. Preheat oven to 350°F (175°C).

2. Spread hazelnuts on a baking sheet and toast for 8–10 minutes, stirring halfway.

3. Remove from oven and let cool slightly. Rub hazelnuts in a clean kitchen towel to remove loose skins.

4. Transfer to a food processor and pulse until finely ground (like coarse sand, not a paste). Set aside.

Step 2: Make the Dough

1. In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy (about 2–3 minutes).

2. Add the egg yolk and vanilla extract. Beat until combined.

3. In a separate bowl, whisk together flour, salt, and ground hazelnuts.

4. Gradually add dry ingredients to the butter mixture. Mix until just combined.

5. Cover and chill the dough for at least 30 minutes to make it easier to handle.

Step 3: Form the Cookies

1. Preheat oven to 350°F (if not already hot). Line baking sheets with parchment paper.

2. Scoop about 1 tablespoon of dough and roll into balls.

3. Roll each ball in granulated sugar and place on the prepared baking sheet, spacing about 2 inches apart.

4. Use your thumb or the back of a ½ teaspoon measuring spoon to gently press a small well into the center of each ball.

Step 4: Bake the Cookies

1. Bake for 12–14 minutes, or until the edges are just starting to turn golden.

2. If the thumbprint puffed up during baking, gently press the center down again right after removing from the oven.

3. Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Step 5: Fill with Chocolate Hazelnut Spread

1. Once cookies are cool, spoon or pipe about ½ teaspoon of chocolate hazelnut spread into each indentation.

2. Optional: Sprinkle with chopped toasted hazelnuts for extra crunch and presentation.

3. Let sit for 10–15 minutes to allow the filling to set, or refrigerate briefly if needed.