Pat the shrimp dry. Use paper towels to remove surface moisture. Dry shrimp sear better and form a better crust.
Mix the spice blend. In a small bowl, combine paprika, garlic powder, onion powder, thyme, oregano, cayenne, black pepper, salt, and brown sugar if using.
Season the shrimp. Toss shrimp with olive oil, then sprinkle the spice mix evenly. Coat all sides so the spices stick well.
Heat the pan. Place a large cast-iron or heavy skillet over medium-high heat.
Add a thin film of oil. The pan is ready when the oil shimmers.
Cook in batches. Add shrimp in a single layer without crowding. Cook 1.5–2 minutes on the first side until the edges look opaque and the underside is darkened.
Flip and finish. Turn shrimp and cook another 1–2 minutes, just until cooked through and springy.
Add butter in the last 30 seconds and toss for a glossy finish.
Brighten and serve. Squeeze lemon over the shrimp. Garnish with herbs. Serve immediately with rice, tacos, salad, or grilled veggies.