Preheat the oven to 325°F. Line 3 baking sheets with parchment paper or silicone baking mats.

Fill a small saucepan with water 1/3 full and bring to a simmer over medium heat. Put the semi sweet and unsweetened chocolate and butter into a heatproof bowl and set it over the water, making sure the bottom of the bowl does not touch the water. Turn off the heat and leave the mixture over the pan to keep hot.

Meanwhile, in a standing mixer with the whisk, whip the eggs, sugar, espresso powder, and vanilla until very thick, about 10 minutes. Gently fold the warm chocolate mixture into the eggs and transfer the mixture to a large mixing bowl .

Put the flour, baking powder, and salt into a fine strainer and sift about 1/3 of it over the chocolate mixture. Gently fold until nearly combined. Sift another 1/3 of the dry ingredients over the bowl and fold; sift the remaining flour over the batter and fold until streaky; add the chocolate chips and fold gently until just combined and no streaks of flour remain. Transfer the batter to a large piping bag fitted with a 1/2-inch plain tip. Pipe rounds of dough about 1.5 inches in diameter about 1 1/2 inches apart on the lined sheets. (you can also use two spoons to do this). Sprinkle some fleur de sel over each cookie.

Bake, rotating the pans halfway through, until just set, 10 to 12 minutes. Let the cookies cool completely on the pan.

Store in an airtight container for up to 3 days.