1. Roast the chiles as for rajas (see Notes) and cut them into ½-inch dice.
2. Heat a heavy pan (preferably cast iron) over medium-high heat until very hot.
3. In a bowl, toss the corn with 1 teaspoon of the olive oil and a sprinkling of salt. Spread the corn in the hot pan and let it blacken slightly, without stirring, for 30 seconds. Have a lid ready in case the kernels begin to pop. Remove the roasted corn from the pan.
4. In the same pan, heat 2 tablespoons of the olive oil. Add the onion and diced chiles and cook, stirring often, until the onion is soft and beginning to brown. Season lightly with salt and remove from the pan.
5. Reduce the heat and add the remaining olive oil. Add the garlic and mushrooms. Cook, stirring often, until the mushrooms are cooked through, 2 to 3 minutes. Return the corn and chiles to the pan and stir to reheat.
6. Turn off the heat and stir in the epazote, if using, black pepper, and queso fresco.
7. To assemble the tacos, spoon some vegetables onto a tortilla. Top with a generous tablespoon of salsa and sprinkle about 1 teaspoon of cotixa cheese over all. Top with a cilantro sprig.
For the Salsa Quemada:
1. Make sure your kitchen is well ventilated. Put a piece of aluminum foil in a heavy sauté pan(preferably cast iron) and set it over medium-high heat. Place the whole tomatoes, chile, and garlic cloves in the pan and dry-roast them on all sides until well charred and soft. The garlic and chiles will be done quickly; the tomatoes may take 10 minutes or longer to cook.
2. Peel the garlic and stem the chile. Place the tomatoes, garlic, and chile in a food processor with the onion, salt, and cilantro. Pulse until the salsa is smooth and taste for seasoning. The salsa will keep, refrigerated, for several days. Reseason before use.