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

Add 2 3/4 cups of all-purpose flour, instant yeast, and salt to the wet ingredients. If using a stand mixer, attach the dough hook and mix on low speed for 2-3 minutes until a shaggy dough forms. If mixing by hand, stir with a sturdy spoon until combined.

Increase mixer speed to medium-low and knead for 6-8 minutes, or until the dough is smooth, elastic, and pulls away from the sides of the bowl. If kneading by hand, turn the dough out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic. The dough should be soft and slightly sticky.

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 2-3 hours, or until doubled in size. The exact time will depend on your kitchen temperature and the activity of your sourdough discard.

While the dough is rising, prepare the cinnamon filling. In a small bowl, whisk together the light brown sugar and ground cinnamon. Set aside.

Lightly grease a 9x5 inch loaf pan and set aside. 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 12x18 inches.

Brush the melted butter evenly over the entire surface of the dough, leaving about a 1/2-inch border on one of the long edges. Sprinkle the cinnamon-sugar mixture evenly over the butter.

Starting from the long edge opposite the plain 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 in half lengthwise. Turn the cut sides up and gently twist the two halves together, keeping the cut sides facing up as much as possible.

Carefully transfer the twisted dough into the prepared loaf pan, tucking in the ends if necessary.

Cover the loaf pan loosely with plastic wrap or a clean kitchen towel and let it rise in a warm place for another 1-1.5 hours, or until it has nearly doubled in size and looks puffy.

Preheat your oven to 350°F (175°C) during the last 15-20 minutes of the second rise.

Bake for 30-35 minutes, or until the top is golden brown and an internal thermometer inserted into the center (avoiding the filling) reads 190-200°F (88-93°C). If the top starts to brown too quickly, you can loosely tent it with foil.

Remove the loaf from the oven and let it cool in the pan for 10-15 minutes before carefully transferring it to a wire rack to cool completely. This is crucial for the bread's texture.

In a small bowl, whisk together the powdered sugar, 2 tablespoons of milk or cream, and vanilla extract until smooth. If the glaze is too thick, add more milk 1 teaspoon at a time until you reach your desired drizzling consistency.

Once the bread has cooled completely (or is just slightly warm), generously drizzle the vanilla glaze over the top. Slice and serve immediately.