If you can’t find tamarind paste (sold in ethnic grocery stores or specialty markets), substitute 1 tsp. lime juice.
Heat skillet coated with cooking spray over medium-high heat. Add potatoes, and sauté 1 minute. Add ginger and jalapeño, and cook 1 minute, or until fragrant. Stir in curry powder, and cook 1 minute more. Add broth, cover, and reduce heat to medium low. Simmer potatoes 10 minutes, or until soft.
Whisk together juice, peanut butter, brown sugar, tamarind paste and salt in bowl. Add to sweet potato mixture, and bring to a simmer. Cook 5 minutes, or until sauce thickens. Serve over rice.