Gently boil potatoes in a large pot of water for 20 minutes, or until tender. Drain, and return to the pot. Mash potatoes with 2 tablespoons butter, 3/4 teaspoon salt, and 1/2 cup milk until fairly smooth. Set aside.
While potatoes are cooking, bring 1 1/2 cups water and 1/2 teaspoon salt to a boil. Stir in kasha. Reduce heat, and simmer, uncovered, for 15 minutes. Add 1 1/2 cups more water and bring to a boil. Add bulgur, cover, and remove from heat. Let stand for 10 minutes.
Preheat oven to 350 degrees F (175 degrees C). Butter a 10-inch pie pan or casserole dish.
In a large saucepan, melt the remaining 2 tablespoons of butter over medium heat. Add onions, garlic, and carrots; sauté until the onions soften. Add mushrooms; cook and stir for 3 to 4 minutes. Sprinkle flour over vegetables; stir constantly for 2 minutes, or until flour starts to brown. Pour remaining 1 1/2 cups milk over the vegetables and increase heat to high. Whisk until sauce is smooth. Reduce heat, and simmer for 5 minutes. Stir in corn, 1/4 teaspoon salt, and black pepper to taste.
Combine vegetable mixture and kasha in a large bowl. Transfer to prepared pan and smooth with a spatula. Spread mashed potatoes over top, leaving an uneven surface.
Bake in preheated oven for 30 minutes. Garnish with the chopped parsley and serve.