Preheat oven to 350° F and line a baking sheet with a silicone baking mat.
In a double boiler, or in a medium glass bowl over a pot of barely simmering water, melt the chopped chocolate and butter over low heat about 3-5 minutes, stirring often. Set aside but keep warm.
In a stand mixer fitted with a paddle attachment, starting on low and slowly raising to medium, beat together the eggs, sugar and vanilla until light and creamy, about 3-4 minutes.
In a small bowl combine the flour, espresso powder, baking powder and salt. Whisk to combine.
Slowly add the flour mixture to the beaten eggs with the mixer on low for 1 minute. Scrape down the sides and with the mixer running on low, pour in the melted chocolate. Stir until everything is fully incorporated but do not overwork the batter, about 30-45 seconds.
Let mixture sit for 5 minutes to firm up.
Using a small 1 tbsp. portion scoop, scoop the cookies and place on the prepared baking sheet, about 6-8 per tray. If they spread out they need more time to firm up. Be sure to leave a 1-inch border around each cookie.
Sprinkle with the Fleur de Sel.
Bake one sheet at a time on the middle rack for 8-10 minutes or until the tops looks cracked and shiny.
Remove the cookies from the sheet and let cool fully on a cooling rack.