Combine dry ingredients (flour, sugar, salt) in a large mixing bowl.
In a separate bowl, combine the sourdough starter, egg and milk.
Add wet ingredients to the large bowl of dry ingredients and mix until crumbly.
Add warmed butter (not hot) to the crumby dough and combine until a sticky dough is formed.
Cover bowl with a damp towel and let dough rest for 15 minutes.
After resting, knead dough with your hands for about 5 minutes, then form into a tight bowl and let ferment at room temperature for 8 hours (or over night).
After fermenting, make the cinnamon roll filling by combining softened butter, cinnamon, and salt in a small bowl.
With your hands, gather the risen dough back and fold back into a ball, then roll dough out onto a lightly floured work surface to form a 12 x 9.5 inch rectangular shape.
Carefully spread cinnamon sugar filling mixture over rectangle with a spoon.
Roll into a long 12 inch log, then cut into 10 even pieces (about 1.2 inches wide each) with a sharp knife.
Generously grease a 9 x 12 baking dish with butter, then place the rolls swirl side up into evenly into the greased pan.
Cover baking dish with a damp towel and place either at room temperature or in a slightly warm spot (like in the oven with the light on) for about 1-2 hours, or until the cinnamon rolls have doubled in size.
Alternatively, cold proof for a slower final fermentation by covering the baking dish with plastic wrap and placing in the refrigerator for 8-24 hours. If they haven't doubled in size in the refrigerator by the time you are ready to bake them, take them our of the refrigerator and let them proof in a warm place until they have.
Preheat oven to 375 Fahrenheit.
Bake cinnamon rolls for about 25 minutes, or until just slightly golden on top. Make sure they aren't overly brown on top or they will bee too dry on the inside! They will continue to bake for a bit once they are removed from the oven.
Remove cinnamon rolls for the oven and make your choice of glaze or frosting recipe while they cool.
*For a naturally sweetened maple glaze, combine maple syrup, butter, vanilla, and salt in a small saucepan set to medium heat. Stir ingredients until butter has melted and ingredients are well combined, then drizzle over cinnamon rolls.
*For a simple vanilla glaze, combine powdered sugar, vanilla, melted butter, milk, and salt with a whisk until well combined. Drizzle over cinnamon rolls.
Once cinnamon rolls have cooled for about 10-15 minutes and you have added your glaze of choice, distribute onto serving dishes and enjoy!