First, sift flour, cocoa powder, and baking soda into a bowl. Mix butter (or margarine) and sugars with a paddle attachment for 3 minutes until fluffy. Mix in the salt and vanilla.
2
After turning off the mixer, add dry ingredients. A few mixer pulses will prevent flour from flying. Next, mix ingredients slowly. speed until combined. Dough forms large, moist pieces.
3
Finish by mixing in the chocolate pieces until combined.
Ensure the crumbly dough is on a clean surface, as it may appear sandy.
4
Split the dough. Press the first portion of dough into a ball and roll it into a 1 1/2-inch diameter (Dorie says length doesn't matter). Plastic wrap aids log rolling.
5
Wrap dough logs in plastic wrap and refrigerate for 3 hours or freeze for 2 hours.
6
Remove the dough logs from the fridge and cut one. Circularly slice 1/2-inch rounds with a sharp knife. If they break while slicing, reassemble them (chips ruin perfect slices).
7
Place cookies 2-inches apart on the baking sheet.
For extra salt and sparkle, sprinkle fleur de sel. If you don't like salt, omit. Repeat with other log.