Heat peanut oil and butter in a stockpot over medium-high heat, add onion, and cook 5 to 7 minutes, or until onions are translucent, stirring often. Add curry powder and cook 3 minutes, stirring constantly, until curry gives off its aroma and is lightly toasted. If necessary, add a tablespoon or two of broth to keep onions from scorching.
Add yams, butternut, carrots, and potatoes, and enough stock to cover vegetables. Bring stock to a simmer, then cover stockpot and cook 1 1/2 hours over low heat, adding stock as needed to keep vegetables covered. Uncover the pot, stir well to break up the potatoes, and cook an additional 30 minutes without adding any more stock. During the final minute of cooking, add cilantro and stir well, then salt to taste.
Serves 6