Preheat oven to 400 degrees. Cover potatoes with water by 2 inches in a medium stockpot. Add 1 tablespoon salt, and bring to a boil. Reduce heat; simmer, partially covered, until tender, about 20 minutes. Drain. Mash potatoes or pass through a ricer. Stir in 4 tablespoons butter, the milk, 1 teaspoon salt, and 1/4 teaspoon pepper. Stir in yolks.
Melt 4 tablespoons butter in a large skillet over medium-high heat. Add leeks, 1 1/2 teaspoons salt, and 1/4 teaspoon pepper. Cook, stirring occasionally, for 2 minutes. Cover; reduce heat to medium-low. Cook, stirring occasionally, until tender, about 8 minutes.
Stir in flour, and cook for 1 minute. Stir in stock, and bring to a boil. Reduce heat, and let simmer for 2 minutes. Stir in lobster and dill, and divide among four 10-ounce baking dishes. Top each with 3/4 cup potato mixture, leaving a 1/2-inch border, and a butter pat. Garnish with dill sprigs. Bake on a baking sheet until bubbling, about 30 minutes. Let stand for 15 minutes before serving.
This recipe can be doubled, but be sure to add three extra lobsters when you make the lobster stock and meat.