In a small bowl, whisk the mayonnaise, mustard, hot sauce, and 1 Tbs. cold water; stir in the tarragon. Transfer to a small covered container and refrigerate. (The dressing may be made up to 5 days ahead.)
Put about 1 inch of well-salted water in a 16- to 20-quart steamer pot, cover, and bring the water to a rolling boil. Meanwhile, kill the lobsters (optional).
When the pot fills with steam, put in the lobsters: Holding them by the body, arrange them two by two on top of each other, facing in different directions so the steam can circulate around them easily.
Cover the pot and steam the lobsters until the shells are bright red and the antennae can be easily pulled off, 10 to 12 minutes for 1-lb. lobsters, 2 minutes longer for each additional 1/4 lb.
Using tongs, transfer the cooked lobsters to a rimmed baking sheet and let sit at room temperature until they’re cool enough to handle. Remove the rubber bands from the claws and then remove the meat from the shells (reserve the shells and bodies for making lobster stock, if you like).
Cut the lobster meat into bite-size pieces. Combine the lobster and celery or cucumber with enough dressing to coat (you may not need all of the dressing). Season to taste with salt and pepper. Cover and refrigerate until cold, at least 30 minutes and up to 4 hours before serving.
When ready to serve, stir the scallions into the lobster salad. Heat a 10-inch skillet over medium heat. Spread the butter on the flat outsides of the hot dog buns. Toast the buns in the skillet, turning once, until golden-brown, about 3 minutes total. Remove from the heat.
Gently open the buns and place a lettuce leaf inside each. Divide the lobster salad among the buns and serve.