Place racks in top and bottom thirds of oven; preheat to 350ºF. Line 2 large baking sheets with parchment and mist with cooking spray.
On a rimmed baking sheet, toast nuts, stirring halfway through, until golden brown, 8 to 10 minutes. Transfer to a bowl to cool. When cool enough to handle, chop finely.
Sift confectioners' sugar, cocoa and salt into a bowl. Stir in egg white and coffee until just smooth. Do not overmix.
Drop teaspoonfuls of dough on lined baking sheets, 2 inches apart. Sprinkle with nuts. Bake, switching sheets halfway through, until puffed and tops are set, about 10 minutes. Let cool completely on sheets on wire racks.
