Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Line 2 baking sheets with parchment paper. Whisk the flour, baking powder and salt in a medium bowl; set aside.
Combine 6 ounces chopped chocolate and the butter in a medium microwave-safe bowl. Microwave in 45-second intervals, stirring after each, until melted, about 2 minutes. Let cool slightly.
Meanwhile, combine the eggs and sugar in a large bowl and beat with a mixer on medium-high speed until very thick and pale, about 5 minutes. Stir the espresso powder and vanilla in a small bowl, then beat into the egg mixture.
Beat the warm chocolate mixture into the egg mixture. Reduce the mixer speed to low and beat in the flour mixture until combined. Beat in the remaining 2 ounces chopped chocolate; finish mixing with a rubber spatula (the dough will be soft).
Drop mounds of dough (1 1/2 tablespoons each) about 2 inches apart on the prepared pans. Bake, switching the pans halfway through, until the cookies are just set around the edges and the tops are puffed and cracked, 10 to 14 minutes. Let cool completely on the pans.