Preheat your oven to 375°F (190°C).
Place the diced potatoes in a large pot and cover them with cold water. Bring to a boil over high heat, then reduce the heat to medium-low and simmer for about 15 minutes, or until the potatoes are fork-tender.
Drain the potatoes thoroughly and return them to the pot.
Add the butter and milk to the pot with the potatoes. Using a potato masher or fork, mash the potatoes until smooth and creamy. Season with salt and pepper to taste.
Transfer the mashed potatoes to a baking dish and spread them out evenly.
If desired, sprinkle shredded cheese, chopped chives, or crispy bacon bits on top of the mashed potatoes for added flavor.
Place the baking dish in the preheated oven and bake for 25 minutes, or until the top is golden and the potatoes are heated through.
Remove from the oven and let cool for a few minutes before serving.
Enjoy your delicious baked mashed potatoes as a comforting side dish with your favorite steak or any main course!