Preheat Oven: Preheat oven to 425°F. Line 2 baking sheets with parchment paper. Pull out a 9x13-inch casserole dish and set aside.
Bake Bacon: While oven is preheating, arrange bacon slices on one prepared baking sheet. Once oven has preheated, transfer that baking sheet to oven and cook bacon until crispy, about 12 to 18 minutes (time will depend on thickness of bacon slices). Remove bacon from oven and set aside to cool. Don’t turn oven off.
Prep and Bake Potatoes: While bacon bakes, prick potatoes several times all over with a fork, then place them on the other prepared baking sheet. Once bacon has finished baking, transfer potatoes to oven and bake until they are tender and can be easily pierced with a fork, about 45 to 60 minutes.
Crumble Bacon: While potatoes cook and once bacon has cooled off, crumble or chop slices into small pieces. Set aside.
Mash Potatoes: After baking, remove potatoes from oven. Don’t turn oven off. Transfer potatoes to a large mixing bowl. Using a potato masher or fork, mash potatoes (it’s OK if some potatoes are still chunky).
Mix in Wet Ingredients and Seasonings: Add sour cream, milk, melted butter, salt, pepper, and garlic powder. Use potato masher or spoon to stir until everything is combined.
Stir In Cheese and Bacon: Stir in half of the shredded cheddar cheese and half of the crumbled bacon. Taste test and add more salt and pepper, depending on taste preferences.
Assemble in Casserole Dish: Coat casserole dish in nonstick cooking spray, then transfer potato mixture to casserole dish and spread into even layer using a rubber spatula. Sprinkle remaining cheddar cheese on top.
Bake: Return casserole dish to oven and bake until the cheese is melted and bubbly, about 10 to 15 minutes. Optionally, transfer casserole dish to top oven rack, turn on oven broiler, and broil until cheese is golden, about 1 to 2 minutes (watch carefully to avoid burning).
Garnish Then Serve: Remove dish from oven and top with remaining bacon crumbles, paprika, and green onions. Allow casserole to cool for a few minutes, then serve.