Step 1
Pour 12 cups water into a large pot. Cut 2 lemons in half and squeeze juice into pot through a sieve or your hand to catch the seeds; toss in spent lemon halves. Season water with kosher salt; bring to a boil.
Step 2
Remove pot from heat and add 1 lb. large shrimp, peeled, deveined. Let sit, uncovered, until shrimp are pink and cooked through, 1–2 minutes. Using a slotted spoon, transfer shrimp to a medium bowl of ice water. Drain shrimp. Coarsely chop, then place in a large bowl.
Step 3
Finely grate zest from remaining 1 lemon to yield 2 tsp., then cut lemon in half and squeeze out 1 Tbsp. juice. Reserve any remaining juice for another use.
Step 4
Heat 4 Tbsp. salted butter, cut into small pieces, and 1 garlic clove, finely grated, in a small microwave-proof bowl in a microwave until butter is melted, about 30 seconds. Pour half over shrimp and toss to coat. (Don’t worry if butter solidifies.) Add lemon zest and juice, 1 celery stalk, finely chopped, ⅓ cup ¾”-long pieces chives, ⅓ cup mayonnaise, and 2 tsp. Dijon mustard and toss to coat. Taste and season with salt if needed. Chill shrimp mixture while you toast the buns.
Step 5
Heat a dry large skillet, preferably cast iron, over medium. Brush insides of 6 hot dog buns, split, with remaining garlic butter. Working in batches, toast buns, cut side down, in pan until golden brown underneath, about 30 seconds per batch.
Step 6
Line insides of buns with 1 small head of butter lettuce, leaves separated, and fill with chilled shrimp mixture.
