In a medium sized mixing bowl, whisk together the all-purpose flour, baking powder, espresso powder, baking soda and salt. Set the dry ingredient mixture aside until needed.
In the bowl of a stand mixer fitted with the beater attachment, or using hand-held electric beaters, beat together the butter, granulated sugar and light brown sugar on medium speed, until light and fluffy, about 5 minutes. Pause as needed to scrape down the bottom and sides of the bowl to ensure thorough mixing.
Add in the eggs, one at a time, beating well after each addition. Again, pause mixing to scrape down the bottom and sides of the bowl. Add in the vanilla bean extract and beat for a further minute, until well combined. The dough should be considerably lightened in colour.
Set the mixer speed to medium-low and add in half the dry ingredients. Beat until roughly combined, then, add in the remainder of the dry ingredients. Beat for a further 30 seconds, or until just combined. Add in the roughly chopped dark chocolate and beat until evenly combined, about 15 more seconds.
Remove the bowl from the stand mixer and cover it in a thin layer of plastic wrap. Set the bowl in the refrigerator to chill for about 30 minutes. If you choose to chill the dough for a longer period of time, see the notes, above.
Meanwhile, pre-heat the oven to 180 Celsius. Line three large baking sheets with non-stick parchment paper. Set aside.
Remove the dough bowl from the refrigerator. Using a small cookie scoop (1” or 2.5 cm) or a generous tablespoon as a measure, scoop out as many cookie balls as possible from the dough. Arrange the dough balls evenly amongst the sheets, then sprinkle over a little of the fleur de sel or flaked salt. You should be able to fit about 8 dough balls per sheet, depending upon size. If you have some dough-balls left-over, either set them aside in the refrigerator to chill whilst the other cookies bake, or seal and store them in the freezer, to bake off later.
Bake, for 9 to 11 minutes, or until the cookies are golden, just firm around the edges and slightly soft in middle. Make sure to rotate the sheets halfway through baking to ensure that the cookies brown evenly. Remove from the oven and let the cookies cool on their baking trays for 10 minutes, before carefully transferring them to a wire rack to cool further, before serving. I prefer to eat ‘em warm.