Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper.
Whisk the flour, cocoa powder, espresso powder, baking powder, baking soda and salt in a medium bowl. Beat the butter and sugar in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 5 minutes. Beat in the eggs one at a time, then beat in the vanilla. Reduce the mixer speed to low; add the flour mixture and beat until just incorporated, then beat in the hazelnuts.
Divide the dough in half and put on the prepared baking sheet. Lightly dust your hands and the dough with flour, then shape each piece into an 11-by-2-inch log, about 5 inches apart. Freeze until firm, about 10 minutes.
Transfer to the oven and bake until puffed and set, 25 to 30 minutes.
Let cool 10 minutes on the baking sheet, then transfer the logs to a cutting board. Reduce the oven temperature to 325 degrees F.
Slice the logs crosswise 1/2 inch thick. Arrange the slices cut-side down on the baking sheet. Return to the oven and bake, turning the cookies halfway through, until dry, about 40 minutes.
Transfer to a rack to cool completely (they will harden as they cool).
