Wash the potatoes, then pat them dry. Dice them into 1/2-inch cubes (if using baby red potatoes, you can slice them in half).
In a large bowl, stir together potatoes, olive oil, garlic powder, kosher salt, and several grinds of fresh ground black pepper.
Line a baking sheet with parchment paper. Spread the potatoes in an even layer on the sheet cut and make sure they are side down. Roast for 15 minutes. Toss with 1/2 cup Parmesan and roast another 15 minutes. Serve immediately, drizzled with sour cream sauce if desired.