Step 1

Combine the warm water, milk, sugar, and yeast in a large bowl. If using active dry yeast, let this sit for around 5-10 minutes until it becomes foamy; if using instant yeast, this step can be skipped.

Step 2

Add the flour, starter, and salt, and use a fork or a wooden spoon to combine it into a shaggy dough. Add the softened butter and work it by hand into the dough, squishing it until combined.

Step 3

Bring the bread dough onto a lightly floured work surface and knead it by hand for around 8-10 minutes until it's soft, smooth, and elastic. This can also be done in a bowl of a stand mixer fitted with a dough hook attachment.

Step 4

Once the dough has been kneaded, place it into a ball in a lightly greased bowl. Cover the bowl with a damp towel and let the dough rise in a warm place until doubled in size, approximately 1 to 1 1/2 hours. The rising time will depend on the room temperature.

Step 5

You can also do this first rise overnight in the refrigerator to give the dough a cold-proof.

Step 6

Punch down the dough and pull it from the bowl onto a lightly floured surface. Line a 9x5-inch bread pan with parchment paper.

Step 7

Flatten the dough with your hands into a rectangle, with the width just slightly shorter than the length of your loaf pan.

Step 8

Orient it so that the longer sides are to your left and right, and the shorter sides are closest and farthest from you.

Step 9

Take the top third of the dough (the edge farthest from you) and fold it halfway back towards the center of the dough. Use your fingers or heel of your hand to press along the fold to seal it.

Step 10

Fold the top (now folded) portion of the dough back over itself again, bringing it back towards the center. Press along this fold to secure the seal.

Step 11

Finally, fold the dough again, bringing the top edge to meet the bottom edge and sealing it.

Step 12

Place the dough seam side down in the lined pan.

Step 13

Loosely cover the dough with a clean kitchen towel or plastic wrap and let the bread rise again until just doubled in size and about 1 inch / 2.5 cm above the rim of the pan. How fast this happens will again depend on room temperature, but the second rise usually takes less time than the first. Take care not to overproof it, especially if your environment is warm.

Step 14

While the dough is rising, preheat the oven to 350°F/180°C.

Step 15

Bake the risen sourdough loaf for about 35 minutes until deep golden brown. If the top is browning too fast before the bread is ready, loosely cover it with aluminum foil after around 20 minutes of baking.

Step 16

After baking, the bread should sound hollow when tapped. The internal temperature of the loaf should be around 200°F/94°C.

Step 17

Let the bread cool to room temperature before slicing.

Step 18

The bread can be stored at room temperature and covered tightly for 4-5 days. Alternatively, it can be frozen for up to 3 months.