Rinse mussels under cold running water; drain well and set aside. Thinly slice serrano pepper. Remove basil leaves from stems; juice lime (2 tablespoons). Whisk together in medium bowl: beer, curry paste, lemongrass, soy sauce, and lime juice until blended.
Heat large saucepot, with tight fitting lid, on medium. Add 2 tablespoons butter, serrano, and garlic to pot. Cook 1–2 minutes, stirring occasionally, until fragrant. Add mussels, beer mixture, and remaining 4 tablespoons butter to pot; raise heat to high. Cover and cook 2–3 minutes until mussels open (discard any unopened shells). Stir in basil; serve with Coconut Rice (see recipe).