Position racks in the lower and upper third of the oven. Line 3 baking sheets with parchment or silicone mats. (If you don't have 3 pans, simply cool the pan between batches.)
Put the butter, unsweetened chocolate and 4 ounces semisweet chocolate in a medium microwave-safe bowl. Heat at 75 percent power in the microwave until soft, about 2 minutes. Stir and heat again until melted, up to 2 minutes more. (Alternatively, put the chocolates and butter in a heatproof bowl. Bring a saucepan filled with an inch or so of water to a very slow simmer; set the bowl over, but not touching the water, and stir occasionally until melted and smooth.)
Stir the granulated and light brown sugars and vanilla into the chocolate mixture with a wooden spoon. Add the eggs and buttermilk and beat vigorously until thick and glossy.
Whisk together the flour, 1/2 cup cocoa and the salt in another bowl. Add them to the egg mixture and stir until just mixed. Stir in the remaining chip-size semisweet chocolate. Cover and refrigerate the dough until firm enough to scoop, about 1 hour.
Roll the dough into scant 1 tablespoon (about 3/4 inch) balls and place 1 inch apart on the baking sheets. Refrigerate until firm, about 1 hour.
Preheat oven to 350 degrees F.
Put the remaining 1/2 cup cocoa powder and the confectioners' sugar in two separate bowls for rolling. Roll the dough balls in the confectioners' sugar, then in the cocoa powder and place on the baking sheets. Bake until the cookies set but are soft and fudgy on the inside, 10 to 12 minutes.
Cool the cookies on the baking sheets for 5 minutes, and then transfer to a rack to cool completely.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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