Step 1

In the bowl of a stand mixer fitted with the dough hook, combine the warm milk and the sugar until dissolved.

Step 2

Add yeast and sourdough starter, stirring to combine.

Step 3

Add flour and sea salt. Mix on medium-low speed until dough comes together.

Step 4

With mixer on low speed, add butter pieces one at a time until combined.

Step 5

Increase speed to medium and let the mixer knead the dough for 5 minutes.

Step 6

Transfer to a large oiled bowl. Cover with oiled plastic wrap and let rise until nearly doubled, about 2 hours if using the optional added yeast. (If using only starter, this proofing time takes 12-18 hours, which yields a beautiful dough with a more pronounced sour flavor.)

Step 7

Turn proofed dough out onto a clean counter top and stretch it several times to develop the gluten. Roll it tightly into a loaf, pinching the seams together. Transfer to an oiled loaf pan and cover with oiled plastic wrap to rise again, about 1 hour or until dough is about 1-inch above the rim of the pan (this step will take a few hours if not using the additonal yeast).

Step 8

Preheat oven to 350°F.

Step 9

Bake at 350° for 35 minutes until golden and internal temperature reaches 190°-200°F on an instant-read thermometer.

Step 10

Remove from oven and immediately brush top with melted butter. (Do not skip this step as it is key to a soft crust.)

Step 11

Let cool in pan for 5 minutes before turning out onto a wire rack to cool completely before slicing.