1/3 cup mayonnaise
2 tablespoons drained capers, finely chopped
1½ tablespoons grated sweet onion (such as Vidalia or Maui)
1 tablespoon chopped fresh tarragon
¼ teaspoon celery seeds
¼ teaspoon (or more) hot pepper sauce
½ cup finely chopped celery heart with leaves

2 tablespoons butter, room temperature
12 small (3-inch round) soft sandwich rolls, split horizontally

1 pound cook, peeled and deveined large shrimp, each cut horizontally in half

Tomato slices
Avocado slices
Thinly sliced butter lettuce

Mix first 5 ingredients and ¼ teaspoon hot sauce in medium bowl. Mix in celery. Season with salt and pepper to taste—and more hot sauce if desired. Cover and chill.

Lightly butter cut sides of rolls. Heat griddle or large nonstick skillet over medium heat. Add rolls, buttered side down, to griddle and toast until golden, 1 to 2 minutes. Spread tarragon mayonnaise over bottom of each bun. Top with shrimp, tomato, avocado, and lettuce. Cover with bun tops. Transfer to plates and serve.