1. 1. Prepare the Dough Base: In a large mixing bowl, combine the sourdough starter discard, warm milk, granulated sugar, egg, melted butter, and vanilla extract. Whisk these wet ingredients together until well combined. The mixture should look smooth and slightly frothy.

2. 2. Combine Dry Ingredients and Form Dough: In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing with a wooden spoon or spatula until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for about 5-7 minutes until it’s smooth and elastic. It should feel soft and spring back when gently poked.

3. 3. Roll Out and Spread Filling: On a lightly floured surface, roll the dough into a large rectangle, approximately 12×18 inches. In a small bowl, combine the softened butter, brown sugar, and ground cinnamon for the filling. Spread this mixture evenly over the entire surface of the dough, going all the way to the edges.

4. 4. Shape the Rolls: Starting from one of the longer sides, carefully roll the dough up tightly into a log. Once rolled, use a sharp knife or unflavored dental floss to cut the log into 12 even slices, about 1.5 inches thick each.

5. 5. Arrange and Bake: Lightly grease a 9×13 inch baking dish. Arrange the cut cinnamon rolls snugly in the prepared dish. Bake in a preheated oven at 375 F (190 C) for 20-25 minutes, or until the rolls are golden brown on top and cooked through. The internal temperature should reach about 200 F (93 C) if you use a thermometer.

6. 6. Whip Up the Frosting: While the rolls are baking, prepare the cream cheese frosting. In a medium bowl, beat together the softened cream cheese and softened butter with an electric mixer until smooth and creamy. Gradually add the powdered sugar, mixing until fully incorporated. Stir in the vanilla extract and 1-2 tablespoons of milk or cream, until the frosting reaches a smooth, spreadable consistency.

7. 7. Frost and Serve: Once the cinnamon rolls are out of the oven, let them cool in the baking dish for about 5-10 minutes. This allows them to set slightly. Then, generously spread the cream cheese frosting over the warm rolls. The warmth will slightly melt the frosting, allowing it to seep into all the nooks and crannies. Serve immediately and enjoy!