Preheat oven to 350F
Cut the sandwich sheet in half through the middle, and brush 1/4 cup melted butter on the cut side.
Place the sandwich sheets, buttered side up, on a cookie tray and bake until lightly browned, about 8 minutes.
Meanwhile, melt 1/4 cup of butter with 2 Tablespoons of water.
Place the butter-water mixture in a blender, along with 1/2 cup cold butter and puree until emulsified.
Pour the butter mixture into the saucepan and add the lobster meat. Cook until heated through, about 5 minutes.
Place the mayonnaise, dijon mustard, mustard powder, 1 teaspoon lemon juice and 1 1/2 teaspoons water in a small bowl and whisk well. Season with salt and pepper, to taste.
Remove lobster from heat and season with the 1 Tablespoon lemon juice, along with salt and pepper.
If using lettuce, place on the bottom rolls, then top with an even layer of buttered lobster, followed by a drizzle of the mustard-mayo sauce and a sprinkling of fresh green onions or chives. Top with top half of the buns.