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1
Cut off kernels from 1 ear corn. Cut remaining 2 ears corn (including cob) crosswise into 1-inch pieces.
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2
Heat oil in a 4-quart heavy pot over moderate heat until hot but not smoking, then cook chiles, garlic, and onion, stirring occasionally, until onion is pale golden, about 7 minutes. Remove from heat and transfer chile mixture to a bowl with a slotted spoon, pressing mixture against side of pot to leave behind as much oil as possible. Do not clean pot; there will be a thin film of oil remaining.
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3
Puree half of chile mixture with 1 cup broth and 2 1/4 teaspoons masa harina in a blender until smooth, then force puree through a medium-mesh sieve with a rubber spatula into a medium bowl.
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4
Puree remaining chile mixture, 1 cup broth, and remaining 2 1/4 teaspoons masa harina in same manner, forcing through sieve into same bowl.
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5
Heat oil remaining in pot over moderately high heat until hot but not smoking, then cook chile pure, stirring constantly, until it reaches a boil and is slightly thickened.
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6
Stir in remaining 4 cups broth and bring to a simmer, stirring occasionally.
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7
Stir in corn (kernels and pieces), mushrooms, green beans, potatoes, and epazote and gently simmer, uncovered, stirring occasionally, until beans are tender, 15 to 25 minutes.
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8
Thin
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9
with a little more broth or water to attain a light cream soup consistency, then stir in salt.
