In a saucepan, over low heat, whisk together the peanut butter, coconut milk, lime juice, curry paste or cayenne, sugar, and salt. Cook, whisking frequently, until heated through, about 5 minutes. Stir in the basil, if desired. Serve warm or at room temperature.
Tip: This rich dipping sauce goes well with grilled chicken, shrimp, pork, or tofu. If desired, sprinkle with black pepper to offset the slight sweetness.