Step 1In a small bowl, whisk together the buttermilk, orange zest and eggs. Set aside.
Step 2In the bowl of a standing mixer fitted with a dough hook, add the flour, sugar, yeast, baking soda, one-fourth teaspoon cinnamon, 1 1/4 teaspoons salt and the softened butter. Pour in the buttermilk mixture and mix on low speed until the mixture comes together, about 1 minute. Increase the speed to medium and continue to mix until the dough is soft and smooth, and pulls away from the sides of the bowl, about 5 minutes, adding a little flour if the dough is too wet (up to one-quarter cup).
Step 3Scrape the dough from the bowl and form it into a ball -- it should be very soft, like brioche dough -- and place the dough in a large, buttered bowl. Cover with plastic wrap and set aside in a warm place until doubled in volume, about 1 hour.
Step 4While the dough is rising, make the filling. In a small bowl, sift together the brown sugar, remaining 2 tablespoons cinnamon, 1 tablespoon flour and 1 teaspoon salt.
Step 5When the dough is risen, remove it to a well-floured work surface. Roll the dough into a rectangle one-fourth-inch thick, and approximately 17 inches long and 12 inches wide. Brush the dough with one-fourth cup melted butter, then sprinkle the cinnamon sugar mixture over it, keeping 1 inch clear along one of the long edges. Loosely roll the dough into a long tube, pressing the clear edge along the length of the dough to seal the tube. Place the tube seam-side down on the cutting board.
Step 6Cut the tube crosswise into 12 evenly sized rolls using a length of unflavored dental floss (slide the floss under the roll and then pull both ends tight, which doesn't squash the dough as much as a knife would). Place the rolls, spiral side up, on a parchment-lined baking sheet, spaced approximately one-half inch apart.
Step 7Cover the rolls loosely with plastic wrap and set aside until almost doubled in size, about 1 hour. Half an hour into the rise, heat the oven to 375 degrees.
Step 8Bake the rolls until puffed and golden brown, about 20 to 25 minutes. Remove from the tray and cool on the parchment paper.
Step 9Meanwhile, make the icing: In a medium bowl, whisk together the powdered sugar, orange juice, milk, remaining 2 tablespoons melted butter and a pinch of salt. The icing should have the consistency of very thick syrup, add additional milk or sugar as needed to adjust the consistency. Ice the rolls by dipping a whisk into the icing and drizzling over the rolls. The rolls can be iced either warm (the icing will melt and spread a bit) or cooled.