The night before you want to eat these make sure you do the following: ensure your sourdough starter is active and bubbly. Then warm butter and milk on stovetop until butter melts. Set aside to cool just slightly.
In a mixing bowl add egg, sourdough starter and sugar. Turn mixer on low speed and slowly add in cooled butter and milk mixture until combined. Slowly add in flour, salt and cinnamon for dough.
Once dough forms, cover bowl with warm towel and allow dough to rest for 30 minutes. Then remove cover, use dough hook to mix on low for 5-6 minutes.
Remove, knead yourself 4 times and place inside a lightly greased dutch oven or bowl, cover with lid or towel and let sit overnight.
The morning you want to eat these do the following: The dough should've risen quite a bit in the 8-12 hours by now.
Put dough on lightly floured cutting board and press down in middle to remove air bubbles. Let dough rest for 15 minutes.
In a bowl combine cinnamon and sugar for filling. When dough has rested, roll it out into a rectangle that is about 1/2" thick all the way across.
Spread dough with softened butter using hands or a spatula. Then sprinkle cinnamon and sugar mixture on top.
Roll this into a long log. Using a serrated knife slice into rolls about 1.5" thick. Place rolls into a cast iron pan or non stick baking dish.
Let rolls rise covered for 60-90 minutes until they've almost doubled in size
Preheat oven to 350 degrees and bake for 35-40 minutes or until they're lightly browned on the top.