Place beans and 6 cups of the water in a large saucepan. Bring to a boil over high; remove from heat, cover, and let stand about 30 minutes. Drain beans, and transfer to a 6-quart slow cooker.

Heat oil in a large skillet over medium-high. Add onion, bell pepper, celery, and garlic; cook, stirring occasionally, until vegetables are beginning to brown, about 6 minutes. Stir in cumin, ancho chile powder, and coriander; cook, stirring constantly, until spices are lightly toasted and fragrant, about 1 minute. Add beer and cocoa, and cook until reduced slightly, about 5 minutes, stirring and scraping to loosen browned bits from bottom of skillet. Transfer mixture to slow cooker.

Add remaining 3 3/4 cups water and bay leaf to slow cooker. Scatter sweet potato chunks over bean mixture; do not stir. Cover and cook on LOW until beans and sweet potatoes are tender, 8 to 9 hours.

Remove 2 cups soup from slow cooker, and place in blender, leaving as many sweet potato chunks in slow cooker as possible. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth; return pureed mixture to slow cooker. Stir in salt and vinegar. Remove bay leaf, and discard. Serve chili with sour cream, cilantro, and avocado.