Carefully melt semi-sweet chocolate and set aside at room temperature.
In a large mixing bowl or bowl of a stand mixer fitted with a paddle attachment, cream together the butter and sugars and salt on high for 3-5 minutes until light and fluffy.
Beat in egg yolks and vanilla.
Add flour slowly with mixer on stir. Or add in 4 parts, mixing until just combined between additions.
Divide dough in half. To half still in mixer, stir in cocoa powder and melted chocolate until fully combined.
Form each dough into long rectangles that are 9x2 inches and about 1 inch thick. Stack them. Wrap with plastic and refrigerate for 2 hours or up to 2 days.
Preheat oven to 325℉ with racks in upper and lower thirds of the oven. Line two or three baking sheets with parchment paper or silpat mats.
Remove the dough from the fridge. Slice longways with a sharp knife. Rotate one half so that the chocolate parts are catty corner from each other, like a checkerboard.
Slice into 1/4 inch slices and place on a baking sheet about 1 inch apart. Bake for 12-15 minutes, until the edges are lightly browned and cookies are firm on the edges. Let cool on the pan for 5 minutes before removing to cool completely.

Checkerboard Cookies

Tuile Cookies