How to Make It
Using a paring knife, slit each shrimp down the back through the shell; remove the dark intestinal vein. Spread the shrimp open and flatten them out. Arrange the butterflied shrimp in a single layer on a rimmed baking sheet, shell side down.
In a saucepan, cover the garlic with water and simmer until softened, about 20 minutes; add more water if necessary. Drain and let cool, then finely chop.
In a bowl, combine the garlic, butter, serranos, cilantro and lime juice. Brush the shrimp with the mixture; season with salt.
Preheat the broiler. Broil the shrimp for 3 to 4 minutes, or just until cooked through. Transfer the shrimp to a platter and drizzle with any remaining butter. Serve with lots of paper napkins.