In small saucepan, combine bacon and cold water to cover. Cover, bring to boil over moderate heat, and skim foam from surface. Reduce heat and simmer 7 minutes. Drain and cool, then cut into 1-inch chunks. Set aside. In 6-quart heavy stock pot over moderate heat, melt butter. Add onion and saute, stirring, until softened, about 3 minutes. Add potatoes and saute 2 minutes. Add stock, bay leaves, salt, and pepper and bring to boil. Reduce heat to moderately low, cover, and simmer until potatoes are soft, about 8 to 10 minutes. Add cabbage and simmer 5 additional minutes. Discard bay leaves. Working in 3 batches, in blender puree soup until smooth (using caution when blending hot liquids). Return to pot, stir in bacon, and rewarm if necessary. Ladle soup into bowls and serve.