In a saucepan, render the bacon for 5 minutes. Add the onions, celery, carrots, and peppers. Saute the vegetables for 5 to 6 minutes or until the vegetables start to wilt. Season with salt and pepper. Stir in the garlic, tomatoes, bay leaves, and chili powder. Saute for 1 minute. Add the beans and chicken stock. Bring the liquid to a boil and reduce to a simmer. Simmer for 2 to 2 1/2 hours, or until the beans are tender and creamy. Remove the beans from the heat. Using a hand-held blender, puree the beans until smooth. Add a little water if the beans are too dry. Strain the sauce through a fine mesh strainer. Discard the strained mixture. Reseason the sauce with salt and pepper if necessary.
Yield: about 1 1/2 quarts of sauce