If using live lobsters, steam or boil them. The boiling ratio is 7 minutes for the first pound and 3 additional minutes for every pound after. Let cool at room temperature. Use a cleaver to crack and remove the meat from the claws, knuckles, and tails. Remove and discard the cartilage from the claws and the intestines from the tails of the cooked meat. Cut the meat into 1/2-inch pieces.
Place diced cucumber in a colander for at least 5 minutes to drain the excess liquid. Combine the lobster, cucumber, mayonnaise, and tarragon (or chervil). If the salad is to be served within the hour, add the scallions (or chives). If not, add them 30 minutes before serving. Season with salt, if needed, and pepper. Cover with plastic wrap, and chill for at least 30 minutes before serving, or as far ahead as the night before.
When you're ready to party: Preheat a griddle over medium-low. Taste the lobster salad—does it need more salt? Does it want more mayonnaise? Now's the time to fix it.
Lightly butter the insides of the buns. Griddle them for about 2 minutes, until golden brown. Stuff the toasted rolls with the chilled lobster salad. Stack 'em high and serve 'em hot.