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Preheat your oven to 375°F (190°C) and line a baking dish with aluminum foil.
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Place the cubed potatoes in a large pot, cover with cold water, add a pinch of salt, and bring to a boil over medium-high heat. Cook until tender, about 15–20 minutes.
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Drain the potatoes thoroughly and return them to the pot. Add the milk, butter, and sour cream, then mash until creamy and smooth.
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Stir in the shredded cheddar cheese, seasoning with salt and pepper, and mix until the cheese melts and blends evenly.
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Transfer the mashed potatoes to the prepared baking dish, smoothing the top evenly.
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Sprinkle the Parmesan cheese and breadcrumbs over the surface to form a crunchy crust.
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Bake for 20–25 minutes, or until the topping is golden brown and crispy.
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Remove from the oven, let cool for a few minutes, then garnish with fresh chives if desired. Serve warm.
 
Variations & Tips:
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Mix roasted garlic into the potatoes for a deeper, more complex flavor.
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Try different cheeses like Gruyère or mozzarella to give the dish a fresh twist.
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Add a little heat with chopped jalapeños or a pinch of cayenne pepper.
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For a lighter option, substitute Greek yogurt for sour cream and use reduced-fat cheese.