Preparation

• In a medium bowl, whisk together the all-purpose flour, rye flour, cocoa powder, salt, and baking soda. In a large bowl, with an electric mixer on medium speed, beat the butter and brown sugar until creamy, about 2 minutes. Add the vanilla, and beat to incorporate, scraping down the bowl as necessary. Add the flour mixture and beat until smooth. Then beat in the chocolate.
• Tip out the dough onto a work surface, and roll it into a neat cylinder. Transfer the log to a sheet of parchment paper about 18×13 inches. Fold the parchment over the dough. Place the long side of a ruler against the bottom edge of the log, on top of the parchment. While pressing firmly on the ruler, pull the top layer of the parchment toward you until you have a smooth 2-1/4-inch cylinder.
• Twist the ends to seal. Refrigerate the log for at least 2 hours.
• Preheat the oven to 325°F. Line 2 baking sheets with parchment paper.
• Cut 1/4-inch slices from the cold dough, rotating it often to maintain the round shape. Place the slices about 2 inches apart on the prepared sheets and bake, 1 sheet at a time, until the cookies look set and dry, 10 to 12 minutes.
• Transfer the baking sheet to a wire rack to cool until the cookies have hardened, about 5 minutes, then transfer the cookies to the rack to cool completely.
• Repeat with the remaining slices.
• Store the cookies in an airtight container at room temperature for up to 3 days or in the freezer for up to 1 month.