1. Heat oil in a medium saucepan over medium-high. Add chiles, onion, garlic, 2 teaspoons salt, and 1/2 teaspoon pepper; cook, stirring occasionally, until onion is softened and lightly browned, 5 to 6 minutes. Add tomato paste, chili powder, cumin, and cayenne, if using; cook until fragrant, about 2 minutes. Stir in sweet potatoes and 4 cups water. Bring to a boil, then reduce heat to medium-low and simmer until potatoes are tender, 20 to 25 minutes. Let cool 5 minutes.

2. Transfer chiles, 1 cup sweet potatoes, and 1 cup cooking liquid to a blender and puree until thick and smooth. Return to saucepan along with beans. Cook over medium heat until beans are warmed through, 5 minutes more. Adjust seasoning as desired. Serve, with avocado, scallions, and corn chips.