Step 1

Dissolve salt in 1 quart cold water in large container. Using kitchen shears or sharp paring knife, cut through shell of shrimp and devein but do not remove shell. Using paring knife, continue to cut shrimp ½ inch deep, taking care not to cut in half completely. Submerge shrimp in brine, cover, and refrigerate for 15 minutes.

Step 2

Adjust oven rack 4 inches from broiler element and heat broiler. Combine oil, garlic, coriander seeds, lime zest, annatto, and pepper flakes in large bowl. Remove shrimp from brine and pat dry with paper towels. Add shrimp and cilantro to oil mixture; toss well, making sure oil mixture gets into interior of shrimp. Arrange shrimp in single layer on wire rack set in rimmed baking sheet.

Step 3

Broil shrimp until opaque and shells are beginning to brown, 2 to 4 minutes, rotating sheet halfway through broiling. Flip shrimp and continue to broil until second side is opaque and shells are beginning to brown, 2 to 4 minutes longer, rotating sheet halfway through broiling. Transfer shrimp to serving platter and serve immediately, passing lime wedges separately.