1/2 cup sourdough starter (recipe below)
1 cup buttermilk
2½ cups all-purpose flour, divided
1 tablespoon sugar
1 teaspoon baking powder
3/4 teaspoon salt
1/2 teaspoon baking soda
Melted butter
Recipe
Combine starter, milk and 1 cup of flour in a large glass or ceramic bowl, cover and let sit out overnight in a warm place. (Be sure to replenish your starter by adding 1/2 cup all-purpose flour and 1/2 cup water; let stand in a warm place for several hours or until bubbly; then refrigerate.)
Gently fold in 1 more cup flour. Combine sugar, baking powder, salt & soda with the last 1/2 cup of flour, and knead it lightly into the dough until mixed well. Roll or pat dough out to 1/2 inch thickness. Dip biscuit cutter in melted butter and cut out biscuits; placing biscuits close together in a greased 13x9x2 inch baking pan. Cover and let rest for 30 minutes. Brush with melted butter and bake in preheated 375° oven about 30 minutes or until golden brown. Serve warm. Makes 12 to 14 biscuits.
Sourdough starter:
2 cups unbleached flour
2 cups warm water
1 package active dry yeast
1 tbsp. sugar or honey
(be sure the flour is unbleached or bread flour, because if chlorine has been used to bleach the flour, it can inhibit the development of the benevolent bacteria which makes this such a terrific leavening agent)
Combine ingredients in a large (like a 2-qt) glass or ceramic bowl. Cover loosely with plastic wrap or use a clean cloth & allow to set out at room temperature for several days; mixture will begin to bubble almost immediately. Stir it once a day until the bubbling activity begins to subside (2 to 4 days) - it should be about the consistency of pancake batter (I like mine a little thicker, so I add a little extra flour each time I feed it) and it should have a nice yeasty sour aroma. Stir it again & then refrigerate it until you're ready to bake.
When you're ready to bake, feed the starter another cupful of flour & a cupful of warm water & leave it out again for several hours or overnight. Then take out what you need to bake with and store the rest in the refrigerator for next time. I keep mine in a pint jar in the refrigerator, I try to have about 1½ cups on hand all the time. You can increase/decrease it by adding more or less flour & water when you feed it - just as long as you use roughly equal amounts of flour/water. If you don't bake for quite a long time, you'll need to feed the starter every 2-3 weeks anyhow; just throw out half of it and feed the other half.
The only danger sign I know of in this procedure is if the starter begins to turn color and/or smells really bad. It's supposed to smell sour & yeasty, but if it really STINKS, it could be going bad. And if it begins to turn PINK (or changes to any other color I s'pose!), it's going bad. Then you just need to pitch it and begin again, making sure to use perfectly clean dishes & utensils (no metal).
One other note - avoid contamination of the starter and dough or batter by keeping it covered all the time as it ferments and rises - I use plastic wrap loosely draped over the bowl or bread loaves/biscuits.