Defrost Shrimp - Place frozen shrimp in a colander and run under cold water until no longer icy, about 3 minutes. Alternatively, set them in a bowl and submerge them in cool water until defrosted. Drain well, and then use paper towels to pat dry.
Season - In a medium bowl, combine shrimp with salt and pepper.
Cook the Garlic - Heat a large skillet over low heat. Add 2 tablespoons of butter. Once melted, add the garlic. Stir and cook until fragrant but not browned, about 2 minutes.
Cook the Shrimp - Add the shrimp to the pan in a single layer, and turn the heat up to medium. Cook until they turn pinkish on the bottom and start curling, about 2 minutes. Flip and finish cooking, about 2 minutes.
Make the Sauce - Turn the heat down to low. Make a well in the center of the pan. Add in the lemon juice, zest, 2 tablespoons of butter, and 1/4 teaspoon salt. Whisk until a lightly thickened emulsified sauce forms. Coat the shrimp in the sauce.
To Serve - Garnish with parsley.
Notes
Storing: Refrigerate in an airtight container for up to 3 days.
Reheating: Cover and. microwave on high setting in 15-second intervals until warm.