In a double boiler melt the chocolate, butter and espresso powder over low heat. Set aside but keep warm.
In a stand mixer fitted with a paddle attachment or with a hand mixer, beat together the eggs, sugar and vanilla until creamy, 3-5 minutes.
In a small bowl combine the flour, baking powder and salt. Stir to combine. Add to the egg mixture. Mix to combine and then pour in the melted chocolate. Stir until everything is fully incorporated but do not overwork the batter.
Let mixture sit for a couple of minutes to firm up.
On a silicone baking mat or parchment lined baking sheet, scoop rounded tablespoons of the batter, leaving 1-inch between each cookie. Sprinkle with the Fleur de Sel and bake for 8-10 minutes or until the top looks cracked and shiny.
Remove the cookies from the sheet and let cool fully on a cooling rack.