01 - Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.
02 - Place cubed potatoes in a large pot, cover with cold, salted water, and bring to a boil. Reduce heat and simmer for 15-20 minutes, or until the potatoes are fork-tender. Drain thoroughly.
03 - In a large bowl, mash the hot, drained potatoes with the melted butter, softened cream cheese, and sour cream until the mixture is smooth and creamy.
04 - Stir in the milk, 1 1/2 cups of the cheddar cheese, most of the crumbled bacon (reserving a few tablespoons for the topping), most of the sliced scallions, garlic powder, onion powder, salt, and black pepper. Mix until all ingredients are well combined.
05 - Spoon the potato mixture evenly into the prepared baking dish. Sprinkle the remaining cheddar cheese and the reserved crumbled bacon over the top.
06 - Bake uncovered for 25-30 minutes, or until the cheese is melted and bubbly.
07 - Remove the casserole from the oven and allow it to rest for 5 minutes. Garnish with the extra sliced scallions before serving.