Step 1: Prepare the Oven and Baking Sheets

Begin by preheating your oven to 350°F (175°C). Proper preheating ensures the cookies bake evenly from the moment they enter the oven.

Line several baking sheets with parchment paper. This prevents sticking and promotes even browning on the bottom of the cookies. Having the baking sheets ready before mixing the dough helps streamline the baking process and allows you to work efficiently once the dough is prepared.

---

Step 2: Cream the Butter and Sugar

Place the softened butter in a large mixing bowl along with the granulated sugar. Using a hand mixer or electric whisk, beat the mixture until it becomes light, creamy, and fluffy.

This process is important because it incorporates air into the butter, creating a lighter cookie texture. Continue mixing for a couple of minutes until the sugar is well incorporated and the mixture looks pale and smooth.

The butter should be soft but not melted. If the butter is too cold, it will not cream properly. If it is too warm, the cookies may spread excessively during baking.

---

Step 3: Add the Egg Yolk and Vanilla

Add the egg yolk and vanilla extract to the creamed butter mixture.

Beat again until everything is fully combined and smooth. The egg yolk adds richness and helps create the tender, delicate texture that makes these cookies so appealing. Meanwhile, the vanilla enhances the overall flavor and complements both the buttery cookie base and the chocolate hazelnut filling.

At this stage, the mixture should appear thick, creamy, and silky.

---

Step 4: Combine the Dry Ingredients

In a separate bowl, whisk together the flour, baking soda, and salt.

Mixing the dry ingredients beforehand helps distribute the baking soda and salt evenly throughout the dough. This ensures consistent flavor and texture in every cookie.

Take a moment to break up any flour lumps before moving on to the next step.

---

Step 5: Form the Cookie Dough

Gradually add the dry ingredients to the butter mixture.

Using a spatula, gently fold everything together until a soft dough forms. Avoid overmixing. Once the flour disappears and the dough comes together, stop mixing.

The finished dough should be soft, smooth, and easy to handle. It should hold its shape when scooped but still feel tender.

Overworking the dough can lead to tougher cookies, so mix only until combined.

---

Step 6: Portion the Dough

Using a very small cookie scoop or approximately two teaspoons of dough, scoop portions onto the prepared baking sheets.

Roll each portion lightly between your palms to create neat, round balls. Aim for cookies that are approximately 1/2 inch in size.

Place the dough balls a couple of inches apart on the baking sheets. Although these cookies do not spread dramatically, giving them some space helps them bake evenly.

Keeping the cookies uniform in size ensures they bake at the same rate and pair together neatly when assembled into sandwiches.

---

Step 7: Flatten the Cookies

Pour a little granulated sugar onto a small plate.

Dip the bottom of a drinking glass into the sugar, then gently press down on each dough ball. This slightly flattens the cookies while adding a subtle sparkle and delicate sweetness to the surface.

The sugar coating also helps prevent the glass from sticking to the dough.

You do not want to flatten them completely. A gentle press is all that is needed.

---

Step 8: Add the Hazelnut Topping

Sprinkle a pinch of finely chopped hazelnuts onto half of the cookies.

These cookies will eventually become the tops of the sandwich cookies, so only decorate half of them. Press the hazelnuts lightly into the surface so they adhere during baking.

The chopped hazelnuts add texture, visual appeal, and an extra layer of nutty flavor that pairs beautifully with the Nutella filling.

---

Step 9: Bake Until Lightly Golden

Place the baking sheets in the preheated oven and bake for 8 to 10 minutes.

Keep a close eye on the cookies near the end of the baking time. The edges should be lightly golden while the centers remain pale and tender.

Because these cookies are small and delicate, they can overbake quickly. Removing them at the right moment helps preserve their melt-in-your-mouth texture.

Once baked, remove the trays from the oven.

---

Step 10: Cool Completely

Allow the cookies to rest on the baking sheet for a few minutes before transferring them to a cooling rack.

Let them cool completely before assembling. This step is essential because warm cookies can cause the Nutella filling to melt and become messy.

As they cool, the cookies will firm up slightly while maintaining their tender interior.

Patience during the cooling stage will result in cleaner, more attractive sandwich cookies.

---

Step 11: Prepare for Assembly

Once the cookies are fully cooled, separate them into pairs of similar sizes.

Match one plain cookie with one hazelnut-topped cookie. Creating evenly sized pairs makes the finished sandwiches look more professional and ensures an even distribution of filling.

Arrange all the cookie bottoms on your work surface before adding the Nutella.

---

Step 12: Fill with Nutella

Spread a generous amount of Nutella onto the flat side of each plain cookie.

You can use a butter knife, offset spatula, or small spoon for this task. Spread the filling evenly, leaving a tiny border around the edge to prevent overflow when the cookies are pressed together.

The amount of Nutella can be adjusted according to your preference. A thicker layer creates a more indulgent cookie, while a thinner layer allows the buttery cookie flavor to shine.

---

Step 13: Create the Sandwich Cookies

Place a hazelnut-topped cookie on top of each Nutella-covered cookie.

Press gently until the filling reaches the edges. Be careful not to press too hard, as these cookies are delicate.

The contrast between the buttery cookie, crunchy hazelnuts, and creamy Nutella creates an irresistible combination of flavors and textures.

---

Step 14: Finish and Serve

Arrange the completed sandwich cookies on a serving platter.

For an extra elegant presentation, lightly dust the tops with powdered sugar just before serving. This creates a beautiful bakery-style appearance and adds a subtle sweetness.

Serve immediately or store in an airtight container until ready to enjoy.

These cookies are perfect for afternoon tea, holiday gatherings, cookie exchanges, dessert platters, or simply enjoying with a cup of coffee.