Prepare the crawfish boil: Place water, Old Bay, fish sauce, lemongrass, lemons, garlic, and ¼ cup of the salt in a large stockpot. Cover and bring to a boil over high. Reduce heat to medium-low, and simmer 15 minutes. Add potatoes, and cook until just tender, 15 to 20 minutes. Add remaining ½ cup salt. Increase heat to high, and bring to a rolling boil. Add crawfish and corn; boil until crawfish are bright red, about 8 minutes. Remove from heat, and let stand 15 minutes.

Prepare the butter sauce: Heat ¼ cup butter in a stainless steel saucepan over medium-high heat until foamy. Add lemongrass; sauté 4 to 5 minutes or until very soft, stirring very frequently. Add remaining butter, garlic, salt, and cayenne. Cook until butter is melted and garlic is just softened, 8 to 10 minutes. Remove from heat; stir in lemon juice. Cover to keep warm.

Drain crawfish mixture. Discard garlic, lemons, and lemongrass. Place crawfish, corn, and potatoes in a huge bowl. Toss with half of the butter sauce and cilantro; serve with remaining sauce.