COMBINE the flour, cocoa powder, baking soda and salt in a large bowl. In a stand mixer fitted with the paddle attachment, cream the butter, sugars and vanilla until extremely fluffy, 5 to 7 minutes. Add the flour mixture and mix until a smooth dough forms, then fold in the chocolate. Wrap the bowl with plastic wrap and refrigerate overnight.
PREHEAT the oven to 350º and line 2 sheet pans with parchment paper. Shape the cookie dough into 1¼-ounce (2-tablespoon) balls, then place them 2 inches apart onto the prepared sheet pans. Sprinkle each with a pinch of fleur de sel.
BAKE, rotating halfway through, 11 minutes, then remove from the oven. While they’re still hot, use a 2¾-inch round cookie cutter to make the cookies perfectly round. Let completely cool, then serve.