How to Make It
Preheat oven to 350°F. Beat butter and granulated sugar with a stand mixer fitted with a paddle attachment on medium speed until creamy, about 2 minutes. Add eggs 1 at a time, beating until just combined after each addition. Beat in vanilla.
Stir together flour, cocoa, baking powder, salt, and cinnamon in a medium bowl. Gradually add flour mixture to butter mixture, beating on low speed until just combined. Beat in chopped pecans until just combined.
Lightly dust hands with powdered sugar. Divide dough in half. Shape each dough portion into a 12- x 3-inch log. Arrange logs 2 inches apart on a baking sheet lined with parchment paper.
Bake in preheated oven 25 minutes. Cool on baking sheet 30 minutes. Cut each log diagonally into 1/4-inch-thick slices using a serrated knife (use a gentle sawing motion). Arrange slices side by side on a baking sheet lined with parchment paper.
Return to oven, and bake at 350°F 7 to 8 minutes. Turn slices over, and bake until dry and crisp, an additional 7 to 8 minutes. Transfer to wire racks, and cool completely, about 20 minutes. Store cookies in an airtight container up to 1 week.