1. Activate the Yeast and Start the Dough Base: In a large mixing bowl, combine the warm milk, 2 tablespoons of the granulated sugar, and the active dry yeast. Stir gently and let it sit for about 5-10 minutes until the mixture becomes foamy. This foam indicates your yeast is active and ready to work. Add the sourdough starter discard, melted and cooled butter, vanilla extract, and salt to the yeast mixture. Stir until well combined.

2. Mix in the Flour: Gradually add the all-purpose flour, about a cup at a time, mixing with a wooden spoon or a stand mixer with a dough hook until a shaggy dough forms. Once all the flour is added, continue to mix or knead on a lightly floured surface for about 5-7 minutes, until the dough is smooth, elastic, and no longer sticky. It should spring back slightly when poked.

3. First Rise: Lightly grease a clean bowl with a little butter or cooking spray. Place the dough in the bowl, turning it once to coat the top. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm place for about 1 hour, or until it has doubled in size. The dough should look visibly puffier and lighter.

4. Prepare the Filling and Shape the Rolls: Once the dough has risen, gently punch it down to release the air. On a lightly floured surface, roll the dough into a large rectangle, approximately 12×18 inches. In a small bowl, combine the remaining granulated sugar (1/2 cup minus 2 tablespoons) and the ground cinnamon. Spread 2 tablespoons of softened butter evenly over the rolled-out dough, leaving a small border along one long edge. Sprinkle the cinnamon-sugar mixture over the buttered dough.

5. Roll and Slice: Starting from one of the long edges, tightly roll the dough into a log. Pinch the seam closed to seal it. Using a sharp knife or unflavored dental floss, cut the log into 12 even slices, about 1 1/2 inches thick.

6. Second Rise and Baking: Lightly grease a 9×13-inch baking dish. Arrange the cinnamon roll slices in the prepared dish, leaving a little space between them as they will expand. Cover the dish with plastic wrap or a clean towel and let them rise again in a warm place for another 30 minutes, or until visibly puffy. Preheat your oven to 375 F (190 C) during the last 10-15 minutes of the second rise. Bake for 20-25 minutes, or until the rolls are golden brown on top and cooked through.

7. Prepare the Icing and Finish: While the rolls are baking, prepare the icing. In a small bowl, whisk together the powdered sugar and 2-3 tablespoons of milk or cream until smooth and pourable. Add more liquid a teaspoon at a time if needed to reach your desired consistency. Once the cinnamon rolls are out of the oven, let them cool for 5-10 minutes, then drizzle generously with the prepared icing. Serve warm and enjoy!