Chocolate-Pistachio Biscotti

Cut long, thin strips of zest (avoid white pith) from oranges. Place zest in a small saucepan, add enough water to cover, and bring to a boil over high heat. Then drain off water. Repeat boiling and draining process 2 more times. Set aside. In a medium saucepan, bring 1 1/2 cups water and 1 cup sugar to a boil. Add zest and simmer for 4 minutes. Set aside to cool. Place chocolate chips in a food processor and pulse 3 or 4 times to break up. Add pistachios and pulse 2 or 3 times. In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside. Preheat oven to 350 degrees F. In the bowl of an electric mixer fitted with paddle attachment, beat together butter and remaining sugar on medium speed until light yellow and fluffy, about 3 minutes. Add eggs and beat on medium speed until incorporated, 2 to 3 minutes. Reduce mixer speed to low; using a slotted spoon, add candied orange zest to butter-sugar mixture and beat for 30 seconds. Gradually add three-fourths flour mixture, scraping down sides of bowl as needed. Add chocolate-pistachio mixture and remaining flour mixture, mixing until dough just comes together. Do not overmix. Divide dough into 2 equal pieces and place on a parchment-lined baking sheet. Using damp hands, shape each piece into a 12-inch-long log, 1/2 inch high. Bake logs until firm, about 24 minutes, rotating baking sheets halfway through. Reduce oven temperature to 250 degrees F. Cool logs on baking sheet for 12 to 15 minutes. Transfer logs to a cutting board. Using a serrated knife, slice each into 24 half-inch-thick biscotti. Place biscotti on parchment-lined baking sheets, spacing 1/2 inch apart. Sprinkle with fleur de sel, if desired. Bake biscotti until slightly crisp, about 10 minutes, rotating baking sheets halfway through. Transfer to a wire rack to cool.