In the bowl of a stand mixer fitted with the paddle attachment, cream together the brown sugar, butter, vanilla and orange zest until light and fluffy.

Add the eggs one at a time, mixing on low speed to combine each one before adding the next. Increase the speed to medium and continue creaming until the ingredients are fully incorporated.

In a separate bowl, sift together the flour, cocoa powder and baking soda. Add the salt.

With the mixer on low speed, slowly add the sifted ingredients to the butter mixture in thirds, scraping down the bowl before each addition. Add the warm melted chocolate and mix slowly until it is fully incorporated. Then add the chopped chocolate and mix just until fully incorporated.

Remove the dough from the bowl and roll it into 2 logs, each about 2 inches in diameter and 10 inches in length. Chill in the fridge for 45 minutes, so the logs hold their shape when slicing.

While the dough chills, preheat the oven to 325°F. Line a baking sheet with parchment paper.

Once the dough has chilled, slice each log into 20 pieces, each about 1 inch thick. The dough will still be a bit soft but should hold together.

Place some granulated sugar on a plate. Fully coat each cookie with sugar, then transfer to the lined baking sheet. Once all the cookies have been coated, bake them for 8 to 10 minutes. Remove from the oven and let cool on the baking sheet.