Heat oven to 375 degrees F. Whisk together the flour, cocoa powder, baking soda and salt in a medium-size bowl.
Coarsely chop 6 ounces of the chocolate, and finely chop the remaining 6 ounces. Microwave coarsely chopped chocolate with butter for 1 minute, stirring halfway through. Microwave an additional 15 seconds if necessary; stir until smooth.
Transfer chocolate mixture to bowl of an electric mixer and add sugars, eggs and vanilla; beat until well combined Reduce speed to low and gradually beat in flour mixture. Stir in remaining finely chopped chocolate pieces.
Drop heaping tablespoonfuls of dough onto ungreased baking sheets, about 2 inches apart.
Bake at 375 degrees F. for 9 to 11 minutes or until set. Let the cookies cool on baking sheet on a wire rack for 2 minutes. Transfer the cookies to rack; let cool completely.
