Proof yeast in 1/4 cup water. Add sourdough starter and remaining water, and beat until well blended.
Combine flour, powdered milk, salt, cocoa powder, and chocolate chips in separate bowl. Blend thoroughly into yeast mixture, about 2 minutes with an electric mixer or 4 minutes by hand. If using mixer, knead with dough hook about 4 minutes, then finish by hand until taut, about 20 seconds. If blending by hand, knead 8 to 10 minutes, until smooth and elastic. Place in large bowl, and cover with plastic wrap. Let rise until tripled, about 3 hours.
Turn dough out onto lightly floured surface, and knead briefly (if making 2 loaves, divide dough in half). Flatten dough out, then spread peanut butter in center. Roll dough up into oval shape, pinching seams. Pick off chocolate chips on outside. Place on baking sheet sprinkled with cornmeal, and let rise again, covered, in warm place until almost doubled in size, about 30 to 45 minutes.
Preheat oven to 450° F.
Sprinkle dough with flour, and slash loaf before baking. Bake until bottom of loaf sounds hollow when tapped, about 60 minutes for large loaf and 35 to 40 minutes for 2 loaves. Let cool on rack at least 2 hours before slicing.
Proof yeast in warm water. Add remaining water and flour, and mix well in a large container - it may triple in size. Cover. Let rise 6 to 24 hours at room temperature.
Makes nearly 2 cups.
Unused portion can be refrigerated in airtight container for up to 2 weeks.Makes about 2 loaves Source: