Step 1In a medium bowl, whisk together the flour and whole wheat flour and set aside.
Step 2In another medium bowl, using the back of a large spoon or with an electric mixer, beat the butter with the sugar, salt and vanilla until smooth and creamy but not fluffy, about 1 minute. Add the cacao nibs and mix to incorporate. Add the flour and mix just until incorporated. Scrape the dough into a mass and, if necessary, knead with your hands a bit, just until smooth.
Step 3Form the dough into a 12-by-2-inch log. Wrap the log tightly in plastic wrap; refrigerate for at least 2 hours, or preferably overnight.
Step 4Position the oven racks in the upper and lower third of the oven and heat the oven to 350 degrees. Line baking sheets with parchment paper.
Step 5Using a sharp knife, cut the cold dough log into one-fourth-inch slices. Place the cookies at least 1 1/2 inches apart on the baking sheets. Bake until the cookies are light golden brown at the edges, 12 to 16 minutes, rotating the baking sheets from top to bottom and front to back halfway through baking. Allow the cookies to rest on the sheets about 1 minute to firm up, then transfer them to a rack using a metal spatula. Let them cool completely. Store the cookies in an airtight container.