Toast the hazelnuts in the oven at 360°F/180°C until lightly colored. Remove from the oven and rub them gently with a dish towel to remove their skin. Let them cool off completely.
Using a food processor, grind the toasted hazelnuts with the sugar until you obtain a sand-like consistency.
Pour your ground mixture into a bowl and add the flour, corn starch, and salt to it. Add the softened butter and work quickly with your fingertips to form the dough. Once the dough is smooth and uniform, refrigerate it for 30 minutes.
Line 2 small baking trays (they'll need to fit in the fridge!) with parchment paper. Roll the dough into ¾ inch (2 cm) logs and cut them into small pieces that weigh about 5 or 6 grams each (about the size of a nickel). Roll all pieces into small balls and placed them on the baking tray. Ensure they are spaced well as they will get a little bigger while baking.
Refrigerate again for 30 minutes (this will ensure the cookies will keep their dome shape).
Turn oven to 285°F/140°C. Bake one tray at the time for about 18-20 minutes. The cookie should be lightly colored on the bottom. Remove from the oven and place the baking tray on a cooling rack. DO NOT TOUCH cookies until they have cooled off otherwise they'll break. Once they've reached room temperature, taste-test one to ensure it's cooked through. If not, return to the oven for one or two more minutes.
Melt the chocolate on a double boiler. Once it's almost entirely melted, remove from the heat and let it rest for 5 min. Mix with a spatula to ensure it's all melted and let it cool off a bit until it becomes thicker and forms ribbons. Pair each cookie with another one of the same shape/size. With a teaspoon, drop a bit of chocolate on one half. The chocolate should be cool enough to not fall from one cookie, if you turn it 90 degrees (look at the photos above). Assemble Baci di Dama by placing the other half cookie on top. Twist the top part to make the chocolate spread evenly. Your Baci di Dama are ready!
Notes
The temperature and baking time varies according to the oven (convection or fan oven will require less time!). I suggest to test a small batch, first. The cookie should be lightly colored on the bottom. Do not bake longer than 20 minutes or the cookie will become too hard.
Recipe by Very EATalian at https://www.veryeatalian.com/baci-di-dama-ladys-kisses/