1. In a 10- by 15-inch pan, arrange tomatoes, onion, garlic, and chilies in a single layer. Broil about 4 inches from heat until charred, turning as needed, 13 to 15 minutes. Remove vegetables as they char. Cool. Peel garlic; pull off and discard chili skin, seeds, and stems. In a blender or food processor, purée vegetables.
2. Pour purée and oil into a 5- to 6-quart pan over high heat. Stir often until mixture is slightly darker, 5 to 7 minutes.
3. Mix masa flour and broth. Stir into pan along with clam juice.
4. For added flavor, if desired, rub crab butter through a fine strainer into pan.
5. Bring mixture to a boil, stirring; occasionally; cover and simmer gently for 5 minutes.
6. Meanwhile, remove husks and silk from corn. With a heavy knife, cut corn crosswise into 1-inch-wide wheels; if needed, pound back of knife gently with mallet to force blade through cob.
7. Add epazote (if dried, tie in cheesecloth), corn, and crab pieces to pan. Cover and simmer until crab is hot, 5 minutes (if cooked) to 10 minutes (if raw). Remove and discard epazote.
8. Ladle soup into bowls. Season to taste with salt and juice squeezed from limes.