Step 1In a metal bowl set over a simmering pot of water, combine the chopped bittersweet chocolate with the butter. Heat the chocolate and butter, stirring frequently, just until the chocolate is melted and the mixture is smooth. Remove from heat and set aside to cool the melted chocolate (it should cool to the point where it is no longer hot but still melted and not yet thickening), 10 to 15 minutes.
Step 2In a medium bowl, whisk together the flour, baking soda and salt.
Step 3In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, beat the eggs. With the mixer running, rain in the sugar until the eggs are light and fluffy (they should approximately double in volume), 2 to 3 minutes.
Step 4With the mixer running, add in the melted chocolate and the espresso, then the flour mixture, mixing just until combined.
Step 5Gently fold in the dark chocolate chunks using a large spoon or spatula.
Step 6The cookie dough will be very soft; cover and chill the dough until it is thickened and scoopable, at least 1 hour.
Step 7Shortly before baking the cookies, heat the oven to 350 degrees.
Step 8Scoop the chilled dough onto parchment-lined baking sheets. Scoop a generous mound using a large spoon or a 1-ounce (2 tablespoon) ice cream scoop. Space the mounds a few inches apart as the dough will spread as it bakes; you should be able to fit about 6 cookies on each baking sheet.
Step 9Bake the cookies until the tops of the cookies are puffed and crisp and the edges of each cookie are set, about 15 minutes, rotating the sheets halfway through for even baking. The cookies will still be soft in the center as they are removed from the oven but will firm up as they cool.
Step 10Place the sheets on a rack until the cookies are cooled.