1. Whisk together flours and salt in a bowl to combine. Beat together butter and sugar on medium-high speed until light and fluffy, about 4 minutes. Reduce speed to low and gradually add flour mixture, beating until just combined. Add cacao nibs and vanilla; beat until just combined.

2. Transfer dough to a work surface and gently knead a few times to evenly distribute cacao nibs. Form dough into a log roughly 12 inches long and 1 1/2 inches in diameter. Tightly wrap log in parchment or wax paper and refrigerate until firm, at least 1 hour and up to 1 day.

3. Preheat oven to 325 degrees and set racks in upper and lower thirds. Unwrap dough and cut into 1/4-inch-thick rounds. Place rounds 1 inch apart on parchment-lined baking sheets and bake, rotating once halfway through, until lightly golden, 12 to 15 minutes. (Cookies will become firm and crisp as they cool.) Transfer cookies to a wire rack and let cool completely. Cookies can be stored in an airtight container at room temperature up to 3 days.