1. 1. Prepare the Dough Base: In a large mixing bowl, combine your active sourdough starter, warm milk, granulated sugar, instant yeast, and salt. Stir gently until well combined. Add the egg and the 1/4 cup of melted and cooled butter, mixing until fully incorporated. Gradually add the all-purpose flour, one cup at a time, mixing until a shaggy dough forms.

2. 2. Knead the Dough: Turn the dough out onto a lightly floured surface. Knead for about 5-7 minutes until the dough is smooth and elastic. It should spring back when lightly poked. Place the kneaded dough into a lightly oiled bowl, turning it 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 to 2 hours, or until doubled in size.

3. 3. Prepare the Bundt Pan and Coating: While the dough is rising, generously grease a 10-12 cup Bundt pan with butter or non-stick spray. In a shallow bowl, combine the 1/2 cup granulated sugar and 1 tablespoon ground cinnamon for the coating. Melt the remaining 1/2 cup of unsalted butter in a separate small bowl.

4. 4. Shape and Coat the Dough Balls: Once the dough has doubled, gently punch it down to release the air. Turn it out onto a lightly floured surface and divide it into roughly 36-40 small pieces (about the size of a golf ball or a large marble). Take each dough piece, dip it first into the melted butter, ensuring it’s fully coated, then roll it in the cinnamon-sugar mixture until evenly covered.

5. 5. Arrange in the Bundt Pan: Arrange the coated dough balls in an even layer in the prepared Bundt pan. Do not pack them too tightly, as they will expand during the second rise and baking. Continue layering until all the dough balls are in the pan.

6. 6. Second Rise and Preheat Oven: Cover the Bundt pan loosely with plastic wrap or a kitchen towel and let it rise again in a warm place for another 30-45 minutes, or until the dough balls look noticeably puffy and have filled out the pan. Towards the end of this rise, preheat your oven to 350 F (175 C).

7. 7. Bake the Monkey Bread: Place the Bundt pan in the preheated oven and bake for 30-35 minutes, or until the monkey bread is golden brown on top and cooked through. You can check for doneness by inserting a thin skewer into the center; if it comes out clean, it’s ready.

8. 8. Cool and Glaze: Let the monkey bread cool in the Bundt pan for about 5-10 minutes. Then, invert the monkey bread onto a serving plate. While it cools slightly, prepare the glaze: whisk together the powdered sugar, 2-3 tablespoons of milk or water, and vanilla extract until smooth. Drizzle the glaze generously over the warm monkey bread and serve immediately.