In a bowl, or using a stand mixer, quickly mix the cold, diced butter and icing sugar until it has a sandy consistency.
Add the egg, mixing just enough to combine, then sift the flour and cocoa together and add to the rest, kneading just enough to obtain a smooth, compact dough. Wrap it in plastic wrap and let it rest in the refrigerator for at least an hour. This step is crucial.
Once chilled, roll out the dough on a lightly floured work surface to a thickness of about 3-4 mm. Use a round cookie cutter to shape your cookies and transfer them to a baking sheet lined with parchment paper. Bake at 180℃ (350°F) in a preheated conventional oven for 12-15 minutes. Once ready, move them to a wire rack to cool completely.
The Filling
Finely chop the gianduiotti and place them in a heatproof bowl, then heat the cream in a small saucepan until it just begins to simmer. Immediately pour the hot cream over the chopped chocolate and let it sit for a minute to melt.
Using a spatula, begin to stir slowly from the center outwards until the cream becomes smooth and shiny. Let it cool to room temperature until it reaches a spreadable consistency.
Assembly, presentation, and storage
Once both the cookies and ganache have cooled to room temperature, proceed with the assembly: spread a generous amount of cream on half of the cookies and close them with the other half, pressing gently. For an elegant touch, you can dust the cookies with a little powdered sugar or, for a more indulgent treat, carefully dip the edges in chopped hazelnuts while the cream is still soft enough.