Begin by sifting the dry ingredients into a bowl (flour, cocoa powder, and baking soda).
Using a mixer with the paddle attachment, cream the butter (or margarine )and sugars until fluffy and creamy, 3 minutes. Next, add in the salt and vanilla and mix to combine.
Turn the mixer off, and add in all the dry ingredients. Give a few pulses of your mixer (you don’t want the flour mixture to fly everywhere). Next, turn the mixture to low speed, begin to combine the ingredients. speed until just combined. The dough will begin to form large, moist pieces of dough.
Finally, add the chocolate pieces and mix, again, just until combined.
Turn the crumbly dough out onto a clean work surface (keep in mind, this will look like a very crumbly mixture…almost sandy!).
Divide the dough into two pieces. Press the first portion of dough into a ball then roll into a 1 1/2-inch in diameter (Dorie says doesn’t matter how long the dough roll is, just make sure you get the diameter right). Wrap with plastic wrap and you could use the wrap to help you roll into your log.
Seal the plastic wrap tightly around dough logs and place in the refrigerator to chill for a full 3 hours or in the freezer for 2 hours (or keep overnight in the refrigerator).
Bake:
When ready to bake, heat the oven to 325F. Line a baking sheet with parchment paper (or a silicon mat).
Remove the dough logs from the refrigerator and begin to cut the first log. Use a very sharp knife and slice circles 1/2-inch thick rounds. Piece them together if they break up while slicing (the chips do mess up the perfect slices you think you’ll be getting).
Arrange the cookies 2-inches apart on the baking sheet.
Add a little sprinkle of fleur de sel for sparkle and if you would like a little extra touch of salt. Omit if you're not into the salty flavor.
Repeat process with the other log.
If you use two baking sheets to bake the cookies, keep one sheet in the fridge with the cookie slices on it while you wait to bake them. Bake one sheet of cookies at a time.
Bake for 12 minutes. If you chose to freeze the dough, bake for 13 minutes. Do not open the door to check on the cookies.
Transfer cookies to a wire rack to cool down. Serve warm or at room temperature. They do firm up more while they cool down.