From Cook's Illustrated. There's an omission in this recipe: step 2 says to remove the salt pork and put in a bowl but they're never referenced again. It's possible their only purpose is to provide the cooking fat, but in that was the case it doesn't seem like it would have been necessary to "dice very small", and usually in that situation instructions say to "remove and discard", not "remove and set aside". Plus, their intro notes start by saying "Salt pork gave our refried beans recipe an authentic flavor"; if the only purpose of the salt pork is to provide cooking fat then it seems like they would have said so or at least phrased that differently in the intro notes. I'm guessing the salt pork is supposed to be added back in at the end. As I've said in other places, I think the people at Cook's Illustrated quite often simply forget to proofread their recipes because I find a lot of dumb mistakes. Anyway here are their notes:
Salt pork gave our refried beans recipe an authentic flavor; onions, garlic, chiles, and cumin gave it complexity; and cilantro and lime juice added at the end gave the beans brightness. If you wish to use dried beans, you can find our basic bean recipe, (see related recipe). Substitute reserved cooking water for chicken broth (you may need to add another 2 to 3 tablespoons of cooking water or broth in step 3 if the beans appear too stiff).
1/2 cup low-sodium chicken broth
2 (15.5 ounces each) cans pinto beans, drained and rinsed
1 tablespoon vegetable oil
3 ounces salt pork, rind removed and diced very small
1 small onion, chopped fine (about 3/4 cup)
1 jalapeno chile, seeds and ribs removed, chile minced
1 poblano chile, seeds and ribs removed, chile chopped fine (about 1/4 cup)
1/4 teaspoon table salt
3 small cloves garlic, minced or pressed through garlic press (about 1 1/2 teaspoons)
1/2 teaspoon ground cumin
1 tablespoon minced fresh cilantro leaves
2 teaspoons fresh lime juice (optional)
1. Process broth and all but 1 cup of beans in food processor until smooth, about 15 seconds, scraping sides of bowl with rubber spatula if necessary. Add remaining beans and process until slightly chunky, about ten 1-second pulses.
2. Heat oil in 12-inch nonstick skillet over medium heat until shimmering. Add salt pork and cook, stirring occasionally, until fat has rendered and pork is well browned, about 10 to 15 minutes. Transfer pork to small bowl with slotted spoon and set aside (you should have about 2 tablespoons of fat left in skillet.)
3. Increase heat to medium-high; add onion, chiles, and salt and cook, stirring occasionally, until softened and beginning to brown, about 5 minutes. Add garlic and cumin; cook, stirring frequently, until fragrant, about 30 seconds. Add beans and stir until thoroughly combined. Reduce heat to medium and cook, stirring occasionally, until beans are thick and creamy, about 5 minutes. Stir in cilantro and lime juice, if using, and serve.