Peel and chop the russet potatoes into uniform pieces.
Place the potatoes in a large pot and cover with water. Bring to a boil over medium-high heat and cook for about 15-20 minutes, or until fork-tender.
Drain the potatoes and return them to the pot.
Add cream cheese, sour cream, milk, garlic powder, onion powder, salt, black pepper, and cayenne pepper (if using) to the potatoes. Mash until creamy and well combined.
Stir in 3/4 cup of shredded cheddar cheese and crumbled bacon (if using) into the mashed potatoes, mixing well.
Preheat the oven to 350°F (175°C).
Transfer the cheesy potato mixture to a greased 8x8 inch baking dish. Spread evenly.
Top with the remaining 1/4 cup cheddar cheese.
Bake in the preheated oven for 15 minutes, or until heated through and the cheese is bubbly.
Remove from the oven and let it cool slightly. Garnish with sliced green onions before serving.