To mix the sourdough cinnamon roll dough, combine the softened butter, whole milk, active sourdough starter, honey, salt, and half of the flour in the bowl of a stand mixer. Turn to low speed and allow to mix until combined, adding the remaining flour as it mixes. Knead on low for about one minute (do not overknead). Dough will not have pulled from the sides of the bowl, but this is fine.
Remove the dough to a greased medium mixing bowl and cover with plastic wrap or damp towel. Allow to sit at room temperature for 8-12 hours (depending on how warm your room is), or until dough is puffy.
To make the cinnamon brown sugar filling, use an electric hand mixer to mix the softened butter, brown sugar, cinnamon, and pinch of salt until well combined. Store at room temperature until ready to assemble the cinnamon rolls.
When ready to assemble, roll the sourdough cinnamon roll dough out on a lightly floured surface until it reaches a 16-inch by 12-inch large rectangle. Spread the cinnamon brown sugar filling evenly over the rectangle.
Roll the dough up, starting with one long side and working towards the other long side. Once rolled up, pinch the long edge of the dough to the roll to help the cinnamon rolls not unwind/separate. Using thread or dental floss, cut the cinnamon roll dough log into 12 individual equal-sized rolls and place in a prepared 9×13 inch baking dish. Cover and allow to rise at room temperature for 1-2 hours.
When the cinnamon rolls have almost finished rising, preheat the oven to 350 degrees Fahrenheit. Once preheated, bake the fresh milled cinnamon rolls for about 30-35 minutes, or until lightly golden. Allow to cool for 10 minutes.
While they cool slightly, add the softened cream cheese and butter to a small mixing bowl and combine until creamy. Add the powdered sugar one cup at a time and mix. Once the frosting is well mixed with no clumps, spread on the slightly warm fresh milled sourdough cinnamon rolls and enjoy.