Combine the buttermilk, sourdough starter, butter, egg, and sugar in a stand mixer equipped with a paddle attachment. Mix until well combined.
⅔ cup buttermilk, ¼ cup salted butter, 1 large egg, ½ cup active sourdough starter, 3 Tablespoons granulated sugar
Add 2 cups bread flour and salt. Mix until a lose dough forms. Add remaining flour and replace the paddle attachment with the dough hook.
2¾-3 cups bread flour, ½ teaspoon salt
Knead the dough until you get an elastic dough that is tacky to the touch but not sticky (it took me about 5 to 6 minutes but each mixer will be different). You should be able to remove a piece of dough and stretch it slightly and see light through without it tearing. (windowpane test)
Once your dough is ready, form the dough into a ball and place into a large, lightly greased, bowl. Cover with cling wrap and set in a warm place and allow to rise for 4 hours or until doubles in size.
Once the dough has risen, roll it out on a lightly floured surface into a large rectangle (about 15x20-inches).
Combine all ingredients for the filling in a medium bowl and use a fork to combine. Use your hands to sprinkle the filling evenly over the dough.
2 Tablespoons butter, 1 tablespoon + 1 teaspoon ground cinnamon, ¼ cup granulated sugar, ¼ cup brown sugar
Starting from the short end, roll the dough as tightly as you can to form a log.
Use a sharp serrated knife (or you can use unflavored dental floss) to cut the log into 8 equal pieces. (I cut the log in half, then cut each of those sections in half, and then again to get as even as possible.)
Slightly grease a 9-inch springform pan and cover the bottom with parchment paper for easy clean up and removal.
Place the cinnamon rolls in the pan and cover with cling wrap and allow them to rise for 1½-2 hours.
After your cinnamon rolls rise, you can choose to place them covered in the fridge to bake the next day or bake immediately. If you choose to bake the next day, place the cinnamon rolls in the refrigerator overnight and bring up to room temperature before baking.
Preheat your oven to 350°F and bake the cinnamon rolls for 20 to 23 minutes checking occasionally to make sure they do not get too brown on top.
Remove from the oven and cool slightly on a wire rack while you prepare the icing.
Add the cream cheese into the bowl of a stand mixer or use a hand mixer and mix on high speed for 2 to 3 minutes until light and fluffy.
4 ounces cream cheese
Add the powdered sugar and vanilla and mix until just combined.
½ cup powdered sugar, ½ teaspoon vanilla extract
Spread over warm cinnamon rolls and serve!