Spray a 9x13-inch casserole dish with nonstick cooking spray.
Peel the potatoes and cut into 1-inch cubes. Place the potatoes in a large Dutch oven and cover by 1-inch water. Add 2 teaspoons kosher salt, 1 teaspoon ground black pepper, 1 teaspoon granulated onion and ½ teaspoon garlic powder; stir well. Set over high heat and bring to a boil, reduce heat to low and simmer until potatoes are fork tender, about 15 minutes.
Drain the potatoes and return to the empty, still hot pot. Set the pot over low heat to boil off excess moisture from the potatoes – this should take just about 1 minute.
Mash the potatoes, and add butter, cream cheese, sour cream, Half and Half and 1 cup cheese. Stir well. Add half of the bacon and 1 tablespoon chopped chives. Stir well. Taste and, if needed, season with additional kosher salt and black pepper.
Spread the mashed potatoes into the baking dish.
At this point, the potatoes can be cooked immediately or refrigerated up to two days.
If baking immediately, adjust oven rack to middle position and preheat oven to 350°F. Cover tightly with foil and cook 15 minutes. Remove foil, and bake another 10-15 minutes until heated through. Sprinkle with remaining 1-cup cheese and bake just until cheese melts. Remove from oven and top with reserved bacon and chives. Serve.
If making ahead, cover tightly with plastic wrap and refrigerate up to 2 days. Remove from the fridge 30 minutes before cooking.
When ready to cook, adjust oven rack to middle position and preheat oven to 350°F. Remove the plastic wrap and cover tightly with foil.
Bake 30 minutes, at 350°F, then remove the foil. Continue baking 20-30 minutes or until heated through. Add reserved shredded cheese and cook just until cheese melts. Remove from oven and top with reserved bacon and chives. Serve, and…