Make the dough: To the bowl of a stand mixer fitted with a dough hook, add the flours, butter, milk, sugar, lemon zest, yeast, cinnamon, orange zest, salt, and 1 cup cool water. Mix on low speed until a cohesive dough begins to form, about 2 minutes, then turn the mixer up to medium and continue mixing until the dough cleans the sides of the bowl, about 2 minutes more.
Lightly flour a clean work surface then turn the dough out onto it. Knead until the dough is smooth, about 1 minute. Grease a large bowl, place your dough inside, and cover loosely with plastic wrap. Set aside at room temperature until doubled in size, 2½–3 hours.
Meanwhile, make the filling: To the bowl of a stand mixer fitted with the paddle attachment, add the butter, brown sugar, and cinnamon. Mix on medium speed until combined and fluffy, 2–3 minutes. Add the pecans and chocolate, mix briefly to combine, then set aside at room temperature.
Lightly flour your work surface and turn the dough out onto it. Using your hands, deflate the dough and shape into a rectangle. Lightly flour the top of the dough, then use a rolling pin to flatten into an even, ¼-inch thick rectangle, about 28- by 16-inches. Using an offset or silicone spatula, spread the filling evenly over the surface of the dough. Working from the longer side, roll into a tight log. Use a large knife to slice the log crosswise into fourteen 2-inch thick buns.
Make the topping: In a small bowl, whisk the cream and brown sugar together, then pour into a 12-inch cast-iron skillet. Sprinkle the pecans and chocolate chunks evenly over the bottom of the pan, then arrange the rolls, cut side down, in the pan (they will fit snugly). Cover loosely with plastic wrap and set aside in a warm place until the rolls have expanded, and feel very soft and jiggly, 2½–3 hours.
Meanwhile, preheat the oven (with one of its racks positioned in the center) to 350°F.
When the buns have risen, uncover and transfer the rolls to the oven. Bake until evenly deep golden brown, 40–45 minutes. Cool in the skillet for 5 minutes, then place a large, heatproof plate or cutting board atop the buns and carefully invert. Serve the sticky buns warm or at room temperature.