1. Place warmed milk, sourdough starter, egg and egg yolk, vanilla, and sugar in bowl of mixer and stir to combine.
2. Add flour, salt, and butter and mix with dough hook until a shaggy dough forms. Scrape sides and continue on speed two for 5 minutes. Scrape sides again and mix for another 5 minutes on speed 2. Scrape one final time, cover, and let rise until doubled.
3. Optional: For the first 2 hours, perform stretch and folds every 30 minutes. This helps strengthen the dough but it is not essential to the recipe if you prefer to just leave it and walk away.
4. Allow the dough to bulk ferment, covered at room temperature for 6-8 hours or overnight. Because this dough has butter and egg it is far less likely to over-ferment unless you have a very hot kitchen.
5. Once dough has doubled, you can shape the rolls or what I prefer to do is place the dough in the refrigerator until cold (makes it much easier to shape) or up to 12 hours for some extra fermentation.
6. When ready to shape the rolls, dump dough onto floured surface and roll dough to a large rectangle. The thinner you roll, the more swirls you will have.
7. Mix all filling ingredients in a small bowl until combined and creamy. Spread dough with filling and roll up into a log.
8. Slice log into about 12 even pieces. I go for 1.5 to 2 inches per cinnamon roll.
9. Place slices into a 9x13 baking dish and allow to rise until puffy which will take about 2-3 hours at room temperature. You can speed this up slightly if you place them in a warmer place like the oven with light on or a proofing box.
10. Optional: Place risen rolls in refrigerator overnight to bake the next morning.
11. When ready to bake, preheat oven to 350 degrees F and bake rolls in center of oven for 35-45 minutes. Tent with foil if they are browning too quickly. The rolls should reach an internal temperature between 190-200 degrees F.
12. Prepare frosting while rolls cool for a few minutes and slather the frosting onto warm rolls.

frosting: mix cream cheese and butter until combined and very creamy - I like to use a hand mixer for this. Add powdered sugar and vanilla and continue mixing. Optional: Add milk to thin and mix until you reach your desired texture.