Activate starter the night before. Combine 30 grams of sourdough starter in a clean pint mason jar and add 35 grams of all-purpose flour and 35 grams of water. This will give you a total of 100 grams of active sourdough starter. If you want or need more starter, increase the measurement. Stir to combine the flour, water, and starter until well combined. Let it sit lightly covered at room temperature for 6-12 hours or until it has doubled in size. You can use a layer of cheesecloth secured by a rubber band or the ring of your mason jar lid. (If you keep your starter ready to go, just measure out 100 grams and proceed with the recipe.)
Heat milk, butter, sugar, salt. In a saucepan, add the milk, butter, sugar, and salt. Place over medium-low heat and slowly bring the mixture up to about 90℉. The butter should start to melt at this point. Do not allow the mixture to go over 100℉. If it gets too hot, let it cool before moving on.
Add flour. To the bowl of your electric stand mixer, add the milk mixture and 2 cups of flour. Using the dough hook attachment, stir to combine.
Add starter. Once combined, add the sourdough starter. Stir to combine.
Mix dough. Slowly add the rest of the flour by adding about ⅓ cup at a time, stirring in between to allow the flour to be fully incorporated. Depending on the weather, you will add between 4 and 4 ½ cups. You will know you have enough flour added when the dough is still slightly sticky and the dough is releasing itself from the side of the bowl. Once the flour has been incorporated, set your stand mixer to medium and allow the dough to knead for about 2 minutes. If you make this by hand, you will want to knead for 5-8 minutes. The dough will be soft, smooth, and shiny when ready.
Bulk rise. Turn the dough out onto a clean, dry surface and grease a large mixing bowl. Transfer the dough to the greased mixing bowl, cover it with a towel, plastic wrap, or a plate, and allow it to sit at room temperature for 8-12 hours or until it has doubled in size.
Make the filling. In a medium bowl, add the butter, cinnamon, brown sugar, and salt. Mix the ingredients until they are well combined and resemble a paste. Set aside.
Prepare pan. Prepare your 9x13 baking dish by spraying a small amount of baking spray.
Roll out dough. Roll the dough out into a 12-inch by 18-inch rectangle (double the size of your baking dish)
Spread cinnamon mixture. Spread the cinnamon butter mixture out onto the dough, making sure it is evenly distributed over the entire surface of the dough.
Cut. Using a pizza cutter, cut the dough into 12 equal strips.
Roll. Roll up the strips to create a cinnamon roll and transfer them to the prepared baking dish. Repeat this process until all 12 are in the baking dish.
Rise. Cover and let the shaped rolls rise for 1 hour.
Preheat oven. About 45 minutes into the rise time, preheat your oven to 375℉.
Bake and cool. Bake at 375℉ for 35-40 minutes or until the rolls are golden brown. Remove from the oven and cool for 20 minutes before topping with frosting and serving.
Make frosting. Mix together the room-temperature cream cheese, butter, vanilla, salt, and powdered sugar in the bowl of your stand mixer. If the frosting seems dry, thin out the mixture by adding 1-2 tablespoons of milk.
Serve. Top the rolls with the frosting and serve.