Butterfly shrimp and place in a large bowl. Add garlic, paprika, and cayenne; stir until shrimp are evenly coated. Refrigerate until needed.

Trim most of the crusts off of the bread.

Add 4 tablespoons melted butter to a large skillet over medium heat. Add bread slices and cook until golden brown, 3 to 4 minutes per side. Remove toasted bread to a plate and wipe out any crumbs from the skillet.

Add remaining 2 tablespoons melted butter to the skillet and increase heat to high. When the skillet is very hot and you see the first wisp of smoke, add shrimp and use tongs to spread into an even layer. Sear, without stirring or tossing, for 1 minute.

Sprinkle with lemon zest, then pour in cream and lemon juice. Stir with a spoon to scrape any browned bits off the bottom. Let cream boil and reduce for 1 to 2 minutes, as shrimp finish cooking and sauce thickens.

Turn off the heat and stir in parsley. Season with salt.

Divide shrimp and sauce evenly over each piece of toasted bread.