Preheat oven to 425F.
Add butter to an 8x10 inch baking dish and melt in oven for 4-6 minutes.
Remove dish and add halved potatoes, tossing to coat in butter.
Sprinkle ranch seasoning over potatoes and toss until evenly coated.
Roast for 30-35 minutes until fork tender and browned on cut sides.
Remove from oven and stir in parmesan if using.
Serve immediately.