In a medium mixing bowl, whisk together the flour, baking soda, and salt. In the bowl of a stand mixer fitted with the paddle attachment (or in a large mixing bowl with a hand mixer), beat the butter, brown sugar, cocoa, orange zest, and vanilla on medium speed until well combined, about 4 minutes. Add the eggs one at a time, beating briefly between additions. 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.