Step 1

Combine the sourdough starter, water, and flour and stir until no lumps remain. Let sit in a warm spot (68-75°F / 20-24°C) until it has risen by about 75% and passes the float test, about 6 hours.

Step 2

In a large bowl, using an electric mixer fitted with a hook attachment, add the flour, sugar, salt, water, and leaven, and knead on low speed until it comes together about 3-4 minutes. Scrape down the sides of the bowl halfway through. Let rest for 20-30 minutes.

Step 3

Knead on low speed and gradually add the butter until it is completely incorporated. Once all of the butter is incorporated, increase the speed to high and knead for about 5-6 minutes until the dough is smooth, releases from the sides of the bowl, and passes the windowpane test. It's a soft, smooth, and satiny dough that is slightly tacky to the touch.

Step 4

Lightly spray a straight-sided container with oil and transfer the dough to the container; lightly oil the top of the dough as well. Cover and let rest at room temperature for 12-15 hours until it has at least doubled, is full of small bubbles and wobbles like jellyfish when you gently shake the container.

Step 5

Spray an 8x4-inch (20x10cm) loaf pan lightly with oil or grease with butter and set aside. Also, turn on the light in the oven to create a warm environment for the dough to rise a second time. Between 75-80°F (24-27°C) is ideal.

Step 6

Remove the dough from the container with a dough scraper, place it on a floured work surface, and flour the top of the dough. Roughly shape the dough into a square and deflate the dough with your fingers to pop the medium and large bubbles. That is important to create a fine and soft bread crumb. If needed, use more flour to prevent the dough from sticking, but be careful not to work more flour into the dough as necessary.

Step 7

Then shape the deflated dough into a rectangle that is about 8-inch (20cm) wide and 10-inch (25cm) long. Starting from a short side, roll up the dough tightly and deflate it even further as you roll it up.

Step 8

Transfer the dough to the prepared loaf pan and let rise in the warm oven for about 1.5-2 hours or until it has grown by about 50-75%.

Step 9

Remove the dough from the oven and preheat the oven to 400°F (204°C) for 15-20 minutes.

Step 10

Brush the dough with egg wash and bake for 30-35 minutes until the top is golden brown and it sounds hollow when you knock on the bottom of the sandwich.

Step 11

Remove from the oven, brush immediately with the melted butter and cover with a clean and dry kitchen towel for 20 minutes. Then uncover, remove the sandwich from the pan and transfer to a cooling rack to cool completely. Store in an airtight container at room temperature for up to 4 days.