In the bowl of a stand mixer, combine active sourdough starter and milk, and whisk until smooth.

Add melted and cooled butter, vanilla, eggs, sugar, and salt, then whisk until fully combined.

Using the dough hook attachment, gradually add the flour while mixing on low to medium speed.

Once all the flour is incorporated, knead on speed 2 with the dough hook for 8 to 10 minutes, until the dough no longer sticks to the bowl.

Transfer the dough to a clean bowl, cover with plastic wrap, and let it rest at room temperature for 2 hours, or refrigerate overnight for 8 to 15 hours.

Remove dough from the refrigerator 1-2 hours before rolling it out. Lightly flour your work surface.

Turn the dough onto the floured surface and roll it into a 17×10-inch rectangle.

Butter the bottom and sides of a 9×13 metal baking dish.

Spread 6 tablespoons of softened butter over the dough.

In a small bowl, combine brown sugar and cinnamon, then sprinkle generously over the dough.

Cut the dough into 12 strips using a pizza cutter, roll each strip tightly, and pinch the edges to seal.

Place the rolls in the buttered baking dish evenly spaced.

Cover with a damp kitchen towel and let them proof in a warm room for 2 to 3 hours.

Preheat oven to 400°F when the rolls are nearly done proofing. Bake for 18 to 20 minutes until lightly golden.

In a large bowl, beat the butter and cream cheese together until smooth.

Add vanilla, powdered sugar, and a pinch of salt, mixing on low speed until incorporated.

Increase to high speed and beat for 2 to 3 minutes until the icing is light and fluffy.

Once the cinnamon rolls are out of the oven, let them cool for 5 minutes and then spread the frosting generously over them.