1 ¼ cups (5.6 ounces) all-purpose flour
¾ cup (3 ounces) buckwheat flour
½ pound (2 sticks) unsalted butter, softened
¾ cup sugar
¼ teaspoon salt
1/3 cup cacao nibs
1 ½ teaspoons pure vanilla extract
Whisk the all-purpose and buckwheat flours together in a medium bowl. In a separate medium bowl (or in the bowl of a stand mixer), beat the butter, sugar, and salt together for about a minute with either a spoon or electric mixer. Mix in the nibs and vanilla. Add the flours and mix just until incorporated (I felt a spoon worked well for this part). Scrape the dough into a mass, if necessary knead it with your hands a few times until smooth. Place the dough on a flat surface and shape into a 12 by 2-inch long. Wrap the log and refrigerate for at least 2 hours or overnight.
When ready to bake, position the racks in the upper and lower thirds of the oven and preheat the oven to 350 degrees. Line two baking sheets with parchment paper or silicone baking mats.
Cut the cold dough log into ¼-inch thick slices. Place the cookies at least 1 ½ inches from each other on the baking sheets. Bake until the cookies are just beginning to color at the edges, 12 to 14 minutes, rotating the baking sheets from top to bottom and front to back halfway through the baking. Cool the cookies in the pans on a rack, or slide the liners off to free up the pans; let cookies cool completely.