1cupunsalted butter,room temperature (226g or 8-ounces)
1large egg,plus one egg yolk
For the filing:
2cupsNutella hazelnut spread or chocolate ganache
For the topping:
1cupsemisweet or bittersweet chocolate chips(6 ounces or 170g)
Preheat oven to 325°F. Toast the hazelnuts in a shallow baking pan until fragrant and skins begin to loosen, about 10 minutes. Vigorously rub the nuts in a clean, dry kitchen towel to remove any loose skins (some skins may not come off - that’s okay.) Cool to room temperature.
In a food processor pulse the skinned, toasted hazelnuts with ½ cup of granulated sugar, until finely ground. Add the baking powder, salt and flour. Pulse a few times to combine.
In a large mixing bowl beat the butter until creamy. Add the powdered sugar and beat until blended. Add the vanilla, egg, egg yolk, and espresso powder. Beat on medium speed until combined, scraping down the sides and bottom of the bowl as needed.
Reduce speed to low and add the flour mixture. Beat until combined and no dry flour remains. The dough will be a little sticky. Divide dough in half and form each into an 8-inch round disk. Wrap each disk in plastic wrap and refrigerate until firm, about 1 hour. Dough may be made ahead and refrigerated for up to 2 days. If well chilled, allow the dough to come to room temperature for 15 minutes before rolling out.
When ready to bake, preheat oven to 350°F. Line two baking sheets with parchment paper. Set aside.
Heavily flour a large piece of parchment paper. Using a well floured rolling pin, roll out one disk of dough to ⅛ or ⅙-inch thick. Cut the dough into rounds using a 2 ½-inch cookie cutter. Using a small thin spatula, carefully transfer the cookie rounds to the prepared baking sheet spacing them 1-inch apart. Gather any dough scraps into a small disk and refrigerate.
Bake for 8 to 12 minutes, depending on the thickness, or until the cookies are light brown around the edges. Cool on the pan for 5 minutes, the transfer to a wire rack to cool completely. Repeat with the remaining chilled disk of dough.
Spread a scant tablespoon of Nutella on the bottoms of half the cooled cookies. Top with the remaining cookies, bottom side down facing the filling.
Microwave the chocolate chips in a bowl at 50% power, stirring at 30-second intervals until melted. Scoop the melted chocolate into a small zipper bag. Cut off a tiny corner of the bag and drizzle the chocolate over the tops of the cookies. Allow the chocolate drizzle to set, about 30 minutes.
Store the cookies at room temperature for 2-3 days or in the refrigerator for up to 5 days. If refrigerated, allow the cookies to come to room temperature for 30-minutes before serving.
Makes 32 to 36 sandwich cookies
Don't get carried away with the Nutella filling. It does not 'set-up' or harden like a ganache, and remains creamy. Too much filling might cause the cookie tops to slide around easily.