In a large mixing bowl (or the bowl of a stand mixer), combine the sourdough discard, warm milk, granulated sugar, egg, and 1/4 cup melted butter. Whisk until well combined.

Add 3 cups of all-purpose flour, instant yeast (if using), and salt to the wet ingredients. Mix with a wooden spoon or a dough hook on low speed until a shaggy dough forms. If mixing by hand, turn the dough out onto a lightly floured surface.

Knead the dough for 7-10 minutes (by hand) or 5-7 minutes (stand mixer) until it is smooth, elastic, and no longer sticky. Add a tiny bit more flour if necessary, but avoid adding too much.

Lightly grease the mixing bowl with a little oil. Place the dough in the bowl, turning once to coat. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm place for 1.5-2 hours, or until doubled in size. (Alternatively, for a slower, richer flavor, you can cold ferment overnight in the refrigerator for 8-12 hours.)

Lightly grease a 9x13 inch baking pan.

Once the dough has risen, gently punch it down. Turn the dough out onto a lightly floured surface and roll it into a large rectangle, approximately 12x18 inches (30x45 cm).

In a small bowl, combine the brown sugar and ground cinnamon. Brush the rolled-out dough evenly with 1/4 cup melted butter, leaving a small border free on one of the long edges. Sprinkle the cinnamon-sugar mixture evenly over the buttered dough.

Starting from the long edge opposite the clean border, tightly roll the dough into a log. Pinch the seam closed to seal.

Using a sharp knife or unflavored dental floss, cut the log into 12 equal slices (about 1.5 inches / 3.8 cm thick). Arrange the slices cut-side up in the prepared baking pan.

Cover the pan loosely with plastic wrap or a clean kitchen towel and let the cinnamon rolls rise in a warm place for another 45-60 minutes, or until puffy and almost doubled in size.

Preheat your oven to 375°F (190°C) during the last 15 minutes of the second rise.

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

Remove the pan from the oven and let the cinnamon bread cool in the pan for 10-15 minutes.

While the bread is cooling, whisk together the powdered sugar, 2 tablespoons of milk, and vanilla extract in a small bowl until smooth. If the glaze is too thick, add more milk 1 teaspoon at a time until it reaches your desired drizzling consistency.

Drizzle the glaze generously over the warm cinnamon bread. Serve immediately and enjoy!