Prepare the Mashed Potatoes. Peel and chop the potatoes into evenly sized chunks. Boil for 15 minutes, or until fork-tender, then drain and mash with butter, sour cream, milk, salt, and pepper until smooth.
Layer in a Casserole Dish. Spread half of the mashed potatoes into a greased casserole dish.
Add Toppings. Sprinkle half of the shredded cheddar cheese, crispy bacon bits, and chopped green onions over the mashed potatoes.
Repeat Layers. Add the remaining mashed potatoes on top, followed by the rest of the toppings.
Bake Until Golden and Bubbly. Bake in the oven at 350F for 25 minutes, or until the cheese is melted and bubbly, and the casserole is golden brown.
Garnish and Serve. Garnish with additional cheese, chopped chives, and a dollop of sour cream if desired. Serve warm.