Dry and season the shrimp: Pat the shrimp completely dry with paper towels, then toss them with the salt, black pepper, and smoked paprika. Dry shrimp sear; wet shrimp steam.

Sear the shrimp: Heat the olive oil and 1 tablespoon of the butter in a large skillet over medium-high heat until the butter foams. Add half the shrimp in a single layer, without crowding, and cook 2 to 3 minutes per side, until they're pink, opaque all the way through, and curled into a loose C. Move them to a plate and sear the rest the same way. A pound and a half of jumbo shrimp will not fit in one layer in a single skillet, and a crowded pan boils them.

Cook the garlic gently: Drop the heat to medium-low and add the remaining 4 tablespoons of butter to the same skillet. Once it melts, stir in the garlic and the red pepper flakes if you're using them, and cook for 30 to 60 seconds, until fragrant and pale gold but not browned.

Build the pan sauce: Pour in the lemon juice and scrape up any browned bits stuck to the bottom of the skillet. Let it bubble for about 1 minute, until the sauce thickens slightly and comes together.

Return the shrimp: Add the shrimp back to the pan along with any juices on the plate and toss for about 30 seconds, just long enough to coat them and warm them through.

Finish and serve: Take the pan off the heat and stir in the parsley. Serve right away, spooning the garlic butter over cooked rice, pasta, or torn pieces of crusty bread.