Methods

To make the dough

1. Start with the dough the day before you want to serve the buns. Lightly butter a large bowl. Pour the warm milk into the bowl of a stand mixer. Scatter the yeast onto the milk and let stand until bloomed and foamy, about 5 minutes.
2. Pour in the sugar, egg, vanilla bean paste, orange zest if using, scrape in the sweet potato, flour, and salt. Place the bowl onto the stand mixer with the dough hook attached. Mix on medium-low speed for 10 minutes, stopping and scraping down the bowl and hook periodically.
3. With the mixer still running, drop tablespoons of butter into the dough until all is incorporated. Let the mixer run for 5 minutes more, at which point the dough will be starting to collect around the hook and pull from the sides of the bowl, but not come away cleanly. If it is not holding height, add a little more flour, a tablespoon at a time, and continue to knead until it does. Scrape the dough into the greased bowl. Cover with plastic wrap and proof in a warm, draft-free spot for 1 hour before refrigerating overnight.

To assemble & bake

1. Preheat an oven to 350°F with the rack in the middle position. Lightly grease a 9-by-13-inch metal baking pan and set aside.
2. Scatter the pecan halves over a quarter sheet pan or rimmed baking sheet and bake until darkened in colour, aromatic, and toasted through, 8 to 12 minutes, check them towards the end of the cooking time, and shuffle the nuts around to toast evenly. Let cool, then roughly break into pieces in a mortar and pestle or in a zip-top bag with a rolling pin. Sieve out any powdery nuts and discard. 
3. For the caramel, melt the butter over medium-low heat. As soon as it is liquid, stir in the remaining ingredients, then raise the heat to medium and bring to a boil, undisturbed, for 15 to 20 seconds. Carefully pour syrup into the prepared pan, tilting the pan to coax the caramel into the edges. Scatter the pecans in an even layer over top, and let cool. 
4. To make the filling, stir together the brown sugar and spices in a small bowl. 
5. Remove the dough from the fridge. Tip out onto a lightly-floured work surface. Dust the top of the dough with flour, as well as your rolling pin. Working quickly but gently, roll the tough into a 20-by-9-inch rectangle, about 1/4-inch thick, with the long side parallel to the edge of the counter. As you work, make sure the dough is not sticking, and try to keep edges straight and corners square. (This can be done on floured parchment paper if concerned about sticking and forming the rolls later.)
6. Brush the rolled dough with the melted butter, leaving a thin border on the far long side. Spread the spiced sugar mixture evenly across the surface. Working from the side closest to you, roll the dough into a tight spiral, pinching the end to seal. If using parchment paper, use it to lift and roll the tender dough. Using your hands, even out the log so that it doesn’t taper to the ends. With a sharp knife, trim the ends to just reveal the swirl. Then cut the log into 12 pieces—cut the log in half first, then cut those pieces in half, then finally those four into thirds. Arrange the rolls, spiral up, on top of the nuts and caramel in the pan, in a 3-by-4 grid. Cover loosely with plastic wrap and let rise in that warm, draft-free spot again until about 60% of their original size, 45 to 60 minutes. 
7. Bake in the hot oven until puffed, golden, and there is a noticeable firm bounce when prodded at the center of the pan, 40 to 45 minutes. Rotate the pan once during baking and tent with foil if browning too quickly. The buns are done when their middles register 185°F on an instant-read thermometer. Remove tray from the oven. Run an offset spatula around the edge of the pan, then cool for 5 minutes on a wire rack set over a parchment-lined baking sheet. Carefully invert the pan onto the rack, revealing the buns. Scrape any stuck nuts from the pan back onto the buns where they belong. Sprinkle with flaky salt, if desired.
8. Serve warm or at room temperature. Store in an airtight container at room temperature for a few days, or freeze for up to 1 month.