Put the ancho and guajillo chiles in a medium saucepan with the beef broth and bring to a boil over medium-high heat. Cover, remove from the heat and allow to sit until softened, about 15 minutes. Remove the stems and seeds from the chiles and place the chiles in a blender along with the beef stock. Blend until smooth, then set aside.
Meanwhile, heat the oil in a large Dutch oven over medium-high heat. Add the beef, 1 teaspoon salt and a few grinds of pepper and cook, stirring once or twice, until the beef is browned, about 10 minutes. Remove the beef with a slotted spoon to a paper towel-lined plate. Add the onions to the pot and cook, stirring occasionally, until softened. Add the garlic, chili powder, cumin and oregano and cook until the garlic is fragrant, about 1 minute. Stir in the tomato paste, diced tomatoes, chile puree, beef, 2 teaspoons salt and a few grinds of pepper. Bring to a boil, then reduce to a simmer and cook, covered, stirring occasionally, until the beef is tender and can easily be cut with a fork, about 1 hour.
Add the butternut squash and cook, stirring occasionally, until tender, about 10 minutes. Serve with sour cream, Cheddar and cilantro.
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