If frozen, thaw the shrimp and pat dry. Place them in a large bowl with the buttermilk. Toss to coat and let sit at room temperature for 10 minutes.
1 pound large shrimp, 1 cup buttermilk
Meanwhile, whisk the cornmeal, flour, garlic powder, paprika, salt, pepper, and cayenne in a shallow bowl.
½ cup cornmeal, ¾ cup all-purpose flour, 2 teaspoons garlic powder, 2 teaspoons ground paprika, 1½ teaspoons kosher salt, ½ teaspoon ground black pepper, ½ teaspoon ground cayenne pepper
A few at a time, shake the excess buttermilk off of the shrimp and bread them in the cornmeal mixture. Coat them completely, using your hands as needed to press the breading onto the shrimp.
Heat a large Dutch oven with 3 inches of oil to 350°F. Line a baking sheet with paper towels. In batches, fry the shrimp until golden brown, about 3-4 minutes. Stir the shrimp a little bit as they cook so they don't stick together.
Fresh parsley, Lemon wedges
Remove the shrimp from the oil and place them on the paper towel-lined baking sheet to drain. Season with another pinch of salt, parsley, and serve with lemon wedges.