Instructions

1. Prep : Preheat the oven to 320°F (160ºC). Line two large baking sheets with non-stick parchment paper..
2. Cream : Beat the unsalted butter and granulated sugar together until smooth and creamy.
3. Mix : Add the ground hazelnuts, flour, and salt to the creamed mixture. Combine until a cohesive dough forms, scraping the bottom of the bowl with a spatula to ensure it is fully mixed.
4. Shape : Divide the dough into four equal portions. Roll each portion into a 9-inch (22 cm) log. Cut each log into 25 even slices and roll them into small balls (about the size of a hazelnut in its shell). Note: If the dough is too sticky to roll initially, chill it for 30 minutes first.
5. Chill : Arrange the dough balls on the prepared baking trays, leaving a little room between each. Chill the trays in the refrigerator for 1 hour to prevent spreading
6. Bake : Bake for 15 minutes until pale golden. Remove from the oven, cool on the tray for 5 minutes, then transfer to a wire rack to cool completely.
7. Melt Filling : Place the chopped chocolate in a microwave-safe bowl. Melt in 30-second bursts on low power, stirring after each interval. Stop just before it is fully melted, letting the residual heat finish the job. Let it cool until slightly thickened.
8. Assemble : Spoon half a teaspoon of melted chocolate onto the flat side of an upturned cookie. Let the chocolate set for a brief moment, then top with a matching cookie half to form a sandwich. Allow the chocolate to set completely before serving (place in the fridge if needed).
* Makes 50 Cookies

Notes & Expert Tips

1. *Mandatory Chilling:** Do not skip chilling the shaped dough balls in the refrigerator for a full hour. Because this dough has a high butter content, skipping this step will cause the cookies to melt into flat puddles rather than maintaining their signature round dome shape.
2. **Uniform Sizing:** Keep the dough balls tiny and completely uniform in size. This guarantees they bake evenly and pair up perfectly during final assembly..
3. **Thicken the Chocolate:** Let your melted dark chocolate cool completely at room temperature until it is slightly thick and holds its shape before assembling. If the chocolate is too hot or runny, the cookie halves will slide right off each other.
4. **Nut Substitution:** If ground hazelnuts are difficult to find, you can easily substitute them with an equal amount of finely ground almond flour.
5. **Storage:** Store the finished sandwich cookies in an airtight container at room temperature for up to one week to keep them crisp.