Pat the shrimp very dry with paper towels. Toss with the salt, pepper and smoked paprika.
Heat a 12-inch cast iron skillet over medium-high heat for about 2 minutes. Add the olive oil and 1 tablespoon of the butter.
When the butter foams, add the shrimp in a single layer. Cook 1 to 1½ minutes per side, until pink outside but still slightly translucent in the center. Move them to a plate. Work in two batches if they do not fit without overlapping.
Lower the heat to medium-low. Add the remaining 3 tablespoons butter, the lemon slices and the garlic. Cook, stirring, for 30 to 45 seconds, until the garlic smells fragrant but has not browned.
Stir in the red pepper flakes, honey and lemon juice. Simmer about 1 minute, until the sauce is bubbling, glossy and slightly thickened.
Return the shrimp and any juices from the plate. Toss in the sauce for about 1 minute, just until the shrimp are opaque and curled into a C shape (145°F / 63°C).
Take the skillet off the heat, scatter with parsley and serve right away with crusty bread or rice for the sauce.