Season the shrimp with ½ teaspoon each salt and pepper. In a small bowl, stir together the garlic and lime juice: set aside. In a nonstick 12-inch skillet over medium-high, heat 1 tablespoon of oil until barely smoking. Add half the shrimp in an even layer and cook with­out stirring until deep golden brown, about 2 minutes. Stir, remove the pan from the heat and continue stirring, allowing the pan's residual heat to finish the cooking, until the shrimp are opaque on both sides, another 20 to 30 seconds.

Transfer to a medium bowl. Repeat with 1 tablespoon of the remaining oil and the remaining shrimp, adding them to the first batch.

Allow the empty skillet to cool for about 5 minutes, then add the remaining 1 tablespoon oil,  the garlic-lime mixture and ½ teaspoon salt. Set the pan over medium-low and cook, stirring fre­quently and scraping up any browned bits, until the garlic is softened and golden brown, about 5 minutes.

Add the jalapeño and any shrimp juices, then cook until the chile is softened, 1 to 2 minutes. Taste and season with salt and pepper. Off the heat, add the shrimp, cilantro and butter and stir until the butter has melted and mixed into the sauce.