In the base of a slow cooker, combine broth, beer and vinegar. Whisk in cornstarch until dissolved. Stir in tomato paste, molasses, chili powder, paprika, salt and pepper. Stir in beef, onion, garlic and tomatoes. Cover and cook on HIGH for 6 hours or LOW for 8 hours.
Stir in beans to heat through. Garnish with sour cream, sliced scallions and cheese and serve with cornbread, if desired.