In a stockpot or large saucepan, melt the butter with the olive oil over medium heat. Add the onions, garlic, cumin, and paprika. Cook for 5 minutes, stirring frequently.
Add the flour and stirring constantly with a whisk, cook for 30 seconds. Slowly add the milk, 1/2 cup at a time, while continuing to stir with a whisk so the flour doesn't lump. Add the tortillas, the shaved cobs, and the sweet potatoes, and simmer the mixture over low heat, stirring occasionally, for 20 minutes or until the sweet potatoes are tender.
Add the corn kernels, and let the soup simmer 2 minutes more. Add the salt, pepper, and lime. Remove the shaved cobs, then ladle the soup into bowls and sprinkle it with cilantro, if you like.
Serves 4 large bowls