Step 1: In a large bowl or the bowl of a stand mixer, combine the warm milk, active dry yeast, and a pinch of granulated sugar. Stir gently and let it sit for 5-10 minutes until foamy.
Step 2: Add the sourdough discard, softened unsalted butter, granulated sugar, salt, and egg to the yeast mixture. Mix well until everything is combined.
Step 3: Gradually add the bread flour, one cup at a time, mixing after each addition. Knead the dough using a stand mixer with a dough hook attachment on medium-low speed for 8-10 minutes, or by hand on a lightly floured surface for 10-15 minutes, until the dough is smooth, elastic, and passes the windowpane test.
Step 4: Lightly grease a large bowl with a little oil. Place the dough in the bowl, turning it once to coat. Cover the bowl with plastic wrap or a clean kitchen towel. Let it rise in a warm place for 1.5 to 2 hours, or until doubled in size.
Step 5: While the dough is rising, prepare the cinnamon filling. In a medium bowl, combine the melted unsalted butter, light brown sugar, and ground cinnamon. Mix until it forms a thick paste. Set aside.
Step 6: Once the dough has doubled, gently punch it down to release the air. Turn the dough out onto a lightly floured surface and roll it into a large rectangle, approximately 18x12 inches (45x30 cm).
Step 7: Evenly spread the cinnamon filling over the entire surface of the dough, leaving a small 1/2-inch border on one of the longer edges.
Step 8: Starting from the long edge opposite the clean border, tightly roll the dough into a log. Pinch the seam gently to seal it.
Step 9: Using a sharp, un-serrated knife or dental floss, cut the log into 12 equal pieces, about 1.5 inches thick.
Step 10: Lightly grease a 9x13 inch baking dish. Arrange the cut cinnamon rolls in the prepared dish, leaving a little space between them.
Step 11: Cover the baking dish loosely with plastic wrap or a clean kitchen towel. Let the rolls rise in a warm place for another 30-45 minutes, or until they look visibly puffy and are nearly touching each other.
Step 12: Preheat your oven to 375°F (190°C) during the last 15 minutes of the second rise.
Step 13: Bake the cinnamon rolls for 20-25 minutes, or until golden brown on top and cooked through. If they start to brown too quickly, loosely tent them with foil.
Step 14: While the rolls are baking, prepare the frosting. In a medium bowl, beat together the softened cream cheese and softened unsalted butter until smooth and creamy.
Step 15: Gradually add the powdered sugar, one cup at a time, beating until smooth after each addition. Stir in the vanilla extract. If the frosting is too thick, add milk or cream one teaspoon at a time until you reach your desired consistency.
Step 16: Once the cinnamon rolls are out of the oven, let them cool in the pan for about 5-10 minutes before generously spreading the cream cheese frosting over them. Serve warm and enjoy!