Place oven racks so the oven is divided into thirds.

Preheat oven to 350 degrees.

Sift all-purpose flour, buckwheat flour and cocoa powder together.

In a large bowl, beat together butter, sugar, brown sugar and salt on medium speed until smooth, about 3 minutes. Add egg yolks and beat for another minute, scraping the bowl as needed, then add the vanilla. Turn mixer off, add all of the dry ingredients, and mix on low speed until everything is almost incorporated. The mixture will look crumbly at first, but will start to come together after a few minutes. Add the kasha and chocolate pieces and mix in.

Divide dough in half and form each half into a disk.

Place one disk between two pieces of parchment paper and roll out to 1⁄4 inch. thickness. Peel away top and bottom pieces of parchment paper, then put dough back on top of one piece of paper. Slide onto a baking sheet (this will help prevent the paper from curling during baking). Repeat with the second piece of dough. Sprinkle the dough with the sugar-salt mixture.

Bake for 14 to 15 minutes, rotating the sheets top to bottom and front to back halfway through baking, or until the cookies are set. Slide the entire cookie slab, including the parchment paper, onto the counter. Cut the big cookie into 1 inch wide diamond shapes. Cool on cookie rack.

These cookies will keep covered at room temperature for about 4 days; they can be frozen, well wrapped, for up to 2 months.