Preheat the oven to 350°F. Line cookie sheets with parchment or nonstick baking liners.
In a medium bowl, whisk together the flour, baking soda and salt.
In a large mixing bowl, use a hand mixer to beat the butter, brown sugar, cocoa, orange zest, and vanilla on medium speed until well combined, about 4 minutes. Beat in the eggs one at a time. Add the cooled chocolate and mix until blended, about 1 minute. Add the dry ingredients and mix on low speed until almost completely blended, about 1 minute. Add the chopped chocolate and mix until blended, about 15 seconds.
Shape the dough into 1¼ inch balls with a small ice-cream scoop or two tablespoons.
Pour the granulated sugar into a shallow dish. Dip the top of each ball in the sugar and set the balls sugar side up about 1½ inches apart on the prepared cookie sheets. Bake one sheet at a time until the cookies are puffed and cracked on top, 11 to 12 minutes. Let the cookies cool on the sheet for 5 minutes before transferring them to a rack to cool completely.
Notes
The balls of dough may be frozen for 1 month. Thaw them overnight in the refrigerator before proceeding with the recipe.