In the bowl of a stand mixer, whisk together the sourdough starter, warm milk, granulated sugar, eggs, and melted butter until smooth.
Add the flour, salt, and cinnamon to the wet mixture and stir with a spatula until a shaggy dough forms.
Knead the dough with the dough hook on medium-low speed for 7–10 minutes, or by hand for 10–12 minutes, until smooth, elastic, and only slightly tacky.
Shape the dough into a ball, place it in a lightly greased bowl, cover with plastic wrap or a towel, and let rise at room temperature for 4–6 hours, or until puffy and 50–75% larger.
While the dough rises, make the topping by combining butter, brown sugar, cream, maple syrup, and salt in a small saucepan over medium heat, cooking and whisking until smooth and just beginning to bubble.
Pour the warm topping into the bottom of the greased 9x13-inch pan and sprinkle the pecan halves evenly over it, then set aside to cool.
For the filling, mix the brown sugar and cinnamon in a small bowl, and ensure the filling butter is very soft and spreadable.
After the first rise, turn the dough out onto a lightly floured surface and roll it into a rectangle about 30x40 cm (12x16 inches).
Spread the softened butter evenly over the dough, leaving a 1.5 cm (1/2-inch) border on one long edge.
Sprinkle the cinnamon-brown sugar mixture evenly over the buttered surface, then scatter the chopped pecans on top.
Starting from the long edge opposite the clean border, tightly roll the dough into a log, sealing the seam gently.
Trim the ends if needed, then slice the log into 12 equal pieces using a sharp knife or bench scraper.
Arrange the slices cut-side up over the cooled pecan topping in the pan, spacing them evenly.
Cover the pan tightly and refrigerate overnight or 6–8 hours for a slow, cold proof.
In the morning, remove the pan from the fridge and let the buns proof at room temperature for 60–90 minutes, or until noticeably puffy and nearly touching.
Preheat the oven to 180°C (350°F) while the buns finish proofing.
Bake the buns for 25–30 minutes, or until golden brown on top and an internal temperature of about 93°C (200°F) is reached.
Let the pan cool on a rack for 5–10 minutes, then carefully invert onto a serving platter so the sticky pecan topping is on top.
Allow the buns to set for another 5 minutes before serving warm.