In the bowl of a stand mixer (or a large mixing bowl if kneading by hand), combine the warm milk, sourdough discard, instant yeast, and granulated sugar. Stir gently and let sit for 5-10 minutes until slightly foamy, indicating the yeast is active.

Add the egg, melted and cooled butter, and salt to the wet ingredients. Mix well.

Gradually add 3 cups of the all-purpose flour, mixing on low speed with the dough hook until just combined. Add the remaining flour 1/2 cup at a time until the dough comes together and pulls away from the sides of the bowl. It should be slightly sticky but manageable. (You may not need all 4 cups of flour).

Increase speed to medium and knead for 8-10 minutes with the dough hook until the dough is smooth, elastic, and passes the windowpane test. If kneading by hand, knead on a lightly floured surface for 10-15 minutes.

Lightly grease a large bowl. Transfer the dough to the bowl, turning once to coat. Cover the bowl with plastic wrap or a clean kitchen towel. Let rise in a warm place for 1 to 2 hours, or until doubled in size.

While the dough is rising, prepare the filling. In a small bowl, combine the softened butter, brown sugar, and ground cinnamon. Mix until well combined and spreadable.

Lightly flour a clean work surface. Turn the risen dough out onto the surface and gently punch it down. Roll the dough into a large rectangle, approximately 12x18 inches (30x45 cm).

Evenly spread the cinnamon filling over the entire surface of the dough, leaving a small 1/2 inch border along one of the longer edges.

Starting from the long edge opposite the border, tightly roll the dough into a log. Pinch the seam closed to seal. Use a sharp knife or unflavored dental floss to cut the log into 12 even slices (about 1.5 inches thick each).

Lightly grease a 9x13 inch baking dish. Arrange the cut cinnamon rolls in the prepared dish, leaving a little space between each as they will expand. Cover the dish with plastic wrap or a clean kitchen towel and let rise in a warm place for another 45-90 minutes, or until puffy and almost doubled in size.

Preheat your oven to 375°F (190°C) about 15 minutes before the second rise is complete.

Bake the cinnamon rolls for 25-30 minutes, or until golden brown on top and cooked through. If they start browning too quickly, you can loosely tent them with foil.

Remove from the oven and let cool slightly in the pan while you prepare the 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 sifted powdered sugar, 1 cup at a time, beating until fully incorporated. Add the vanilla extract and 1 tablespoon of milk or cream. Beat until smooth.

Add additional milk or cream, 1 teaspoon at a time, until the frosting reaches your desired smooth, spreadable consistency.

Spread the cream cheese frosting generously over the warm (but not hot) cinnamon rolls. Serve immediately and enjoy!