Blend the flour, cocoa, and cinnamon; set aside. With the paddle of an electric mixer (or regular beaters), cream the butter on medium speed until soft and creamy but not melted. Add the salt and confectioners’ sugar; mix on medium-low speed until thoroughly combined, about 5 minutes, scraping the bowl as needed. Reduce the speed to low and add the egg; mix until blended. Add the walnuts, the pistachios, and the flour mixture; as soon as the dough comes together, stop the mixer. Scrape the dough onto a large sheet of plastic wrap. Using the wrap to help shape and protect the dough, gently press it into a 6-inch square that’s 1-1/2 inches thick. Wrap in plastic and refrigerate until the dough is firm enough to slice, at least 4 hours.
Heat the oven to 400°F. Line a baking sheet with parchment. Unwrap the dough, trim the edges, and slice the square into four 1-1/2-inch square logs. Slice each log into square cookies between 1/8 and 1/4 inch thick. Lay the squares 1/2 inch apart on the baking sheet. Bake until the tops look dry and the nuts look slightly brown-tinged, 8 to 10 minutes, rotating the sheet halfway through. Cool the cookies on the baking sheet until cool enough to handle (about 10 minutes) and then transfer the cookies to a rack.
nutrition information (per serving):
Photo: Mark Ferri