5ouncesbittersweet chocolatechopped into 1/4" pieces (I used a Ghirardelli bar & chopped it)
In a medium bowl, sift together the flour, cocoa powder, and baking soda. Set aside.
In a large bowl (I used my stand mixer), cream the butter. Add in the brown sugar, sugar, fleur de sel, and vanilla. Beat on low until combined.
Add in the sifted dry ingredients and beat on low until just combined. The dough will look crumbly, but should hold together. You don't want to overmix once the dry ingredients have been added.
Knead in the chocolate pieces until evenly distributed.
Move the dough onto a clean work surface (I use a large cutting board) and form the dough into a log (I made two logs), about 1" - 1 1/2" in diameter. Wrap the log(s) in plastic wrap and refrigerate for at least an hour (or up to 3 days). [I put one in the fridge overnight & one in the freezer so it'd be ready for weekend baking]
When ready to bake, preheat the oven to 350 degrees and line baking sheets with either parchment paper or silicone baking mats.
Use a sharp knife to slice the log into 3/8" - 1/2" slices, and place on your baking sheets, about 1" apart.
Bake on the middle rack for 12-15 minutes, until just puffed and cracked on the tops (add an extra minute if you're starting with frozen dough). Let the cookies rest on the baking sheets for about 5 minutes before transferring to a wire rack to cool completely.
The dough should chill for at least one hour but can stay in the fridge for up to 3 days. You can also freeze the dough and bake from frozen.