Melt butter in a large cast iron skillet over medium heat. Add garlic, shallot and red pepper flakes, and cook, stirring frequently, until fragrant, about 2 minutes.
Add shrimp; season with salt and pepper, to taste. Cook, stirring occasionally, until pink and cooked through, about 3-4 minutes.
Stir in parsley, lemon zest and lemon juice.
Serve immediately.
Notes
Use a cast iron skillet. Cast iron skillets retain heat much better than a nonstick skillet, allowing for even cooking and a better sear on the shrimp.
High quality butter goes a long way. Although any butter will work well here, high quality European butter has a higher butterfat content than standard American butter (82% vs 80%), resulting in better emulsification for that richer, silkier butter sauce for the shrimp.
Know your shrimp size. Shrimp size is calculated on number of shrimp per pound. The smaller the number, the larger your shrimp. There are about 40-50 medium shrimp per pound. Note that smaller shrimp will cook much faster!
Buy frozen shrimp. For superior quality and freshness, using frozen shrimp is ideal (but avoid the bags with freezer burn), opting for a quick thaw under cold running water.
Pat the shrimp dry. This is crucial to remove any excess surface moisture for that crispy, golden brown sear. Too much moisture will steam the shrimp instead.
Add freshly squeezed lemon juice (and zest). The lemon will help balance out all the flavors, cutting through the richness of the butter while providing a vibrant, zesty kick.
Cook in batches. Overcrowding the pan will lead to poor searing and chewy, rubbery shrimp.
Mix it up. Add a splash of white wine (pinot grigio or sauvignon blanc), diced tomatoes, leafy greens, or serve over pasta.
Reheat in the oven. Leftover shrimp can be rubbery when reheated in the microwave. For best results with leftovers, reheat in the oven at 325°F, covered in aluminum foil until warmed through.
Serve with crusty bread. Serve with all the homemade crusty bread for dipping, sopping and dunking!