Makes 1 loaf
Make a starter as follows that will keep in the refrigerator for making sourdough bread:
Put the ½ cup of lukewarm water in a bowl. Add yeast and let stand until dissolved, about 10 minutes. Stir.
Add the balance of the ingredients to the first mixture and stir well. Cover and let stand for 3 days in a warm place - ideally, one with a constant temperature of about 78 degrees. Stir the mixture daily.
Measure the starter in a bowl. Stir sugar and butter into the milk. Cool to lukewarm, and add this to the starter.
Stir in the amount flour specified or enough more to make a soft dough. Turn it out on a floured board. Knead it well for a minute or two. Place the dough in a buttered bowl and brush it lightly with melted butter. Cover the bowl with a towel and let the dough rise in a warm place, free from drafts, until doubled, about 1½ hours.
Punch dough down and let rise again for about 30 minutes, or until doubled in bulk.
Punch down once more and shape into a round ball. Let rest for 10 minutes. Shape the dough into a loaf, place in a greased 9 × 5 × 3 pan. Cover with the towel, and again let rise in warm place until doubled in bulk, about 55 minutes.
Bake in a preheated 400° oven about 50 minutes, or until the crust is crisp and well browned.
To continue the starter, add to the remainder of it 1 cup lukewarm water, ½ cup sifted flour and 1 teaspoon sugar. Cover the mixture and refrigerate until you need it to make sourdough bread again.