Step 1 Using a double boiler, or in a bowl set over a pot of simmering water, combine 13/4 cups chopped chocolate with the butter and stir until melted. Set aside to cool. Meanwhile, in a small bowl, whisk together the flour, baking powder and salt.
Step 2 Using a stand mixer, or in a mixing bowl using an electric mixer, whip the eggs and sugar to the "ribbon" stage (the eggs will puff and lighten in color, and when the beater is removed, the beaten eggs and sugar form a slowly disappearing "ribbon" over the surface of the eggs in the bowl), 8 to 10 minutes.
Step 3Slowly add the espresso and vanilla, then the melted chocolate. Add the flour mixture and gently fold in the remaining 2 cups chopped chocolate.
Step 4Chill the mixture until it is firm enough to scoop, at least 1 hour.
Step 5Shortly before baking, heat the oven to 325 degrees. Scoop about 2 tablespoons worth of dough to form each cookie and place on a parchment-lined doubled baking sheet (the extra pan will help to keep the cookies from burning). Bake until the top of the cookie is crisp and shiny, about 20 minutes, rotating halfway through. The cookies will still be soft as they come out of the oven but will firm up as they cool.
Step 6Place the cookies, still on parchment, on a rack until fully cooled.