Preheat your oven to 220°C (425°F). Line a baking sheet with parchment paper or foil.

In a large mixing bowl, toss diced potatoes with olive oil, smoked paprika, garlic powder, onion powder, kosher salt, and black pepper until every piece is coated.

Spread potatoes evenly in one layer on the baking sheet.

Roast in the oven for 35 minutes, turning potatoes halfway through, until lightly browned and crispy.

While potatoes are roasting, make the cheese sauce. In a medium saucepan over medium heat, melt butter. Whisk in the flour and cook for 1 minute.

Slowly whisk in milk, stirring constantly to avoid lumps. Let the mixture simmer for 1-2 minutes until it thickens.

Lower the heat and add cheddar cheese gradually, whisking until smooth and melted.

Add garlic powder, cayenne (if using), and salt; mix well. Keep on very low heat, stirring occasionally, until ready to use.

When potatoes are done, divide them onto plates. Drizzle each serving generously with cheese sauce and add a big spoonful of sour cream on top.

Sprinkle with chopped green onions or chives if desired. Serve immediately.