Place the potatoes into a large pot and cover with salted water. Bring to a boil, then reduce heat to medium-low, cover, and simmer until tender, about 20 minutes. Drain and allow to steam dry for a minute or two. Season the potatoes with salt, and mash with 6 tablespoons of butter, sour cream, egg, and milk.
Preheat oven to 350 degrees F (175 degrees C). Grease a 2-quart casserole.
Heat 1 teaspoon butter in a skillet over medium heat, and cook and stir the cabbage, leeks, and onion until the cabbage is tender and the onion is translucent, about 10 minutes. Crush 2 bouillon cubes into the cabbage mixture, and stir to blend and dissolve the cubes. Stir the cabbage mixture into the potato mixture until thoroughly mixed, and spoon into the prepared casserole.
Bake in the preheated oven for 40 minutes; top with Cheddar cheese, and return to oven until the cheese melts, about 10 minutes.
