Preheat oven to 375°F (190°C).
Grease a baking dish with butter.
Boil diced potatoes for 8–10 minutes until slightly tender and drain.
In a saucepan melt butter and stir in heavy cream and sour cream.
Add garlic powder, onion powder, salt, and pepper and mix well.
Stir in half of the cheddar and mozzarella until melted.
Layer potatoes in baking dish and pour cheese sauce evenly over them.
Top with remaining cheddar and mozzarella cheese.
Bake for 30–35 minutes until golden and bubbly.
Garnish with chopped parsley before serving.