Place first 5 ingredients in a zip top baggie. Use your fingers to mash butter into the other ingredients until uniform. Set aside (can refrigerate day before making casserole).
Peel and halve the potatoes. Place in pot and cover with water. Bring to a boil, then cover and reduce the heat to a simmer for 30 minutes or until tender when pierced with a fork. Drain and mash (can refrigerate day before making casserole).
Stir in 1 cup of sugar/butter topping mix, sugar, vanilla, egg and half and half. Spoon into the buttered casserole dish; top with remaining topping.
Bake at 350 degrees for 45 minutes. Cover with foil if topping starts to brown too much.