These bite-sized buttery, nutty cookies coated generously in confectioners' sugar are a holiday classic. This version is loaded with chopped chocolate and cocoa because we can’t resist that chocolate-hazelnut combo.
Position racks in the upper and lower thirds of the oven and preheat to 350˚ F. Spread the hazelnuts on a baking sheet and bake on the lower rack until lightly toasted, about 10 minutes. Let cool completely, about 20 minutes.
Pulse the hazelnuts in a food processor until finely ground. Add the flour, cocoa powder and salt and pulse to combine.
Beat the butter in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Add 1/2 cup confectioners' sugar and the vanilla; beat on low speed until combined, then increase the speed to medium high and beat until well combined. Reduce the speed to low again, add the flour mixture and beat until combined. Beat in the chopped chocolate. Cover the dough and refrigerate until slightly firm, about 30 minutes.
Scoop heaping tablespoonfuls of dough and roll into balls. Arrange about 2 inches apart on 2 unlined baking sheets. Bake, switching the pans halfway through, until the cookies spread slightly, have a matte finish and look slightly darker around the edges, 18 to 24 minutes. Let the cookies cool 5 minutes on the pans, then transfer to a rack to cool completely.
Put the remaining 1 cup confectioners' sugar in a medium bowl. Gently roll the cookies in the sugar to coat completely, rerolling as needed. Dust the tops of the cookies with cocoa powder.