11tablespoonsunsalted butter156g, room temperature
⅔cuppacked light brown sugar134g
½teaspoonfleur de selor ½ teaspoon kosher salt
1teaspoonpure vanilla extract
5ouncesbittersweet chocolatecoarsely chopped
Sift flour, cocoa powder, and baking soda into a medium bowl.
Add butter to the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until creamy, about 1 minute. Add sugars, salt, and vanilla; beat 2 minutes more, scraping down sides as necessary.
With the mixer off, add dry ingredients. Cover with a clean kitchen towel and mix on low until just combined (to minimize flying flour); uncover and continue to mix on low until just combined (the dough will be crumbly). Add chopped chocolate and mix until just combined.
Line a baking sheet with parchment paper.
Turn dough out onto work surface and gather into a disc, kneading if necessary. Roll into a circle ½-inch thick. Cut with 1 ½-inch diameter biscuit cutter and transfer to prepared baking sheet for chilling - no need to leave space between as you're not baking them yet. Cover with plastic wrap or a second sheet of parchment paper and freeze for 2 hours or refrigerate for 3 hours or overnight.
When ready to bake cookies, preheat oven to 325˚F. Line a baking sheet with parchment paper and arrange half of the cookies about 2 inches apart. Bake 12 minutes (cookies won't look done, this is okay). Transfer baking sheet to a wire cooling rack and let cool until warm or to room temperature before serving.
Repeat with remaining dough if desired.
Refrigerate unbaked cookies for up to 3 days or freeze for up to 2 months. To bake frozen cookies, place in oven straight from the freezer and increase baking time by 1 minute.