In a large mixing bowl, cream together 5 1/2 tablespoons of softened unsalted butter, 1 teaspoon of kosher salt, and 6 tablespoons of granulated sugar until light and fluffy. Beat in 1 large egg, then gradually stir in 3 1/2 cups of unbleached all-purpose flour and 2 teaspoons of instant yeast. Finally, add 1 1/8 to 1 1/4 cups of buttermilk until a soft dough forms.

Turn the dough out onto a lightly floured surface and knead for 10-12 minutes until smooth and slightly tacky. Place in a greased bowl, cover with a cloth, and let it rise in a warm area for about 2 hours, or until doubled in size.

While the dough rises, prepare the filling. In a small bowl, mix 1/4 cup of softened unsalted butter, 2 tablespoons of fresh orange juice, the zest from 1 1/2 oranges (reserve 1/2 teaspoon for the glaze), and 1/2 cup of granulated sugar. Stir until blended and smooth.

Once the dough has risen, punch it down and turn it out onto a floured surface. Roll it into a rectangle approximately 12x18 inches. Spread the filling mixture evenly, then sprinkle 2 ounces of chopped dark chocolate on top. Roll the dough tightly into a log and slice it into 12 equal pieces.

Take each piece and twist it into a figure-eight shape, tucking the ends underneath. Place the twisted rolls into a greased 9x13 inch pan, leaving space between each for expansion.

Cover the pan with a cloth and let the rolls proof at room temperature until doubled in size again, about 30-60 minutes.

Preheat oven to 350°F (175°C). Bake the rolls for 25-30 minutes until golden brown.

While the rolls are baking, whisk together 1 1/2 cups of powdered sugar, 3 tablespoons of fresh orange juice, and the reserved orange zest until smooth.

Once baked, let the rolls cool slightly for about 10 minutes. Drizzle the orange glaze generously over the warm rolls before serving.