In medium bowl, whisk together flour, cocoa, baking soda and salt.
Using an electric mixer, beat butter and sugar on medium speed until light and fluffy, about 3 minutes. Reduce mixer speed to low and beat in vanilla and egg to combine. Add flour mixture, beating until just incorporated.
Divide dough in half; using plastic wrap to guide, roll into 2 logs, about 8 inches long and 11⁄2 inch in diameter. Roll in pistachios to evenly coat, then wrap in plastic wrap. Refrigerate until firm, at least 1 hour.
Heat oven to 350°F. Line 2 baking sheets with parchment paper. Slice dough into 1⁄4-inch- thick rounds and transfer to prepared baking sheets. Bake until tops feel sandy to the touch, 11 to 13 minutes. Cool on sheets 2 minutes, then transfer to wire racks to cool completely.
Nutritional information (per cookie):About 55 calories, 3 g fat (1.5 g saturated), 1 g protein, 20 mg sodium, 6g carbohydrates,
TAHER AGROINDUSTRIAL GROUP WISHES YOU BEST & HOPES ENJOY YOUR CHRISTMAS