Fit a heatproof bowl over a pan of gently simmering water, making sure the water doesn't touch the bottom of the bowl, and put the butter in the bowl. Coarsely chop 6 ounces of the chocolate and scatter it over the butter. Finely chop the remaining 2 ounces of chocolate and set it aside. Leave the bowl over simmering water, stirring occasionally, until the ingredients are just melted, taking care that they don't get so hot that they separate. Remove the bowl from the pan.
Using a whisk or heatproof spatula, stir in the sugar. The mixture will turn grainy, but that is ok. One by one, add the eggs, whisking energetically after each one goes in and then for a minute or two more. The dough will become smoother, shinier, and thicker. Whisk in the vanilla and the salt and then, less vigorously, the flour. Stir in the finely chopped chocolate. Transfer the dough to a bowl, cover and refrigerate for at least 3 hours.
When ready to bake, center a rack in the oven and preheat to 350 degrees F. Line two large baking sheets with parchment paper or silicone baking mats.
Put confectioners' sugar in a small bowl. Using a medium cookie scoop, scoop out level portions of dough or use a tablespoon to get rounded spoonfuls. Roll each into a ball and drop into the bowl of sugar. Gently toss the dough around in the sugar until well coated. Place the ball on one of the baking sheets and repeat, giving the balls about 2 inches of spread space.
Slide the baking sheet in the oven and bake the cookies for 12 minutes, rotating the baking sheet after 6 minutes. The cookies will have spread and cracked, their sides should feel set and their centers should still be a little soft. Put the baking sheet on a cooling rack and wait 2 minutes, then carefully transfer the cookies to the rack using a broad spatula. Let the cookies cool completely.
Repeat with the second baking sheet and the remaining dough.
Note-store cookies in an airtight container on the counter for up to 3 days.