1. 1. Combine Wet Ingredients and Starter: In a large mixing bowl, whisk together the active sourdough starter, warm milk, granulated sugar, room temperature egg, and cooled melted butter until well combined. The mixture should look slightly frothy and uniform.

2. 2. Add Dry Ingredients: Gradually add the all-purpose flour and salt to the wet mixture. Use a wooden spoon or a sturdy spatula to mix until a shaggy dough forms. It will be sticky at this stage, but don’t worry.

3. 3. Knead the Dough: Turn the dough out onto a lightly floured surface. Knead for about 8-10 minutes until the dough becomes smooth, elastic, and less sticky. It should pass the “windowpane test” – you should be able to stretch a small piece of dough thin enough to see light through it without tearing. If you have a stand mixer, use the dough hook attachment and knead on medium-low speed for 6-8 minutes.

4. 4. First Rise (Bulk Fermentation): Place the kneaded dough in a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a damp kitchen towel and let it rise in a warm spot for 4-6 hours, or until doubled in size. Alternatively, for a deeper flavor, you can refrigerate it overnight for 8-12 hours.

5. 5. Shape the Loaf: Gently punch down the risen dough to release the air. Turn it out onto a lightly floured surface. Shape the dough into a rectangular loaf that fits your loaf pan (a standard 9×5 inch pan works well). You can also divide it into three equal pieces and braid them for a beautiful presentation. Place the shaped dough into a lightly greased loaf pan.

6. 6. Second Rise (Proofing): Cover the loaf pan loosely with plastic wrap or a damp towel and let it rise again in a warm spot for 1-2 hours, or until it has nearly doubled in size and looks puffy. It should spring back slowly when gently poked with a finger.

7. 7. Prepare for Baking: Preheat your oven to 375 F (190 C). If using, whisk the egg yolk and water together to create an egg wash. Gently brush the top of the risen loaf with the egg wash for a golden, shiny crust.

8. 8. Bake the Loaf: Bake for 30-35 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C) when measured with an instant-read thermometer. If the top browns too quickly, you can loosely tent it with aluminum foil. Remove the loaf from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely before slicing.