Sift together the flour, cocoa and baking soda in a separate bowl;set aside.
Beat together the butter and both sugars on MED speed for about 3 minutes.
Then beat in the salt and vanilla.
Add flour mixture on LOW speed until big, moist, curd-like dough forms.
Toss in the chopped bitterweet chocolate, and knead within the bowl until it comes together.
Divide the dough in half (weigh if possible), and shape into logs that are 1-1/2 inches in diameter. Compress any weak spots and re-roll.
Wrap each log in plastic.
Freeze for 2 hours OR refrigerate for at least 3 hours before slicing.
Preheat oven to 325 ℉.
Slice into 1/2" rounds. Keep unused dough in fridge.
Place slices about 2" apart on rimmed baking sheet lined with parchment paper.
Bake for 12 minutes on CENTER RACK.
Remove from oven and allow cookies to cool ON THE PAN for a few minutes, otherwise the will fall apart. THEN, transfer cookies to cooling rack.