Preheat the oven to 425. Line a large rimmed baking sheet with parchment paper or foil.
Cut carrots in half crosswise, and then cut lengthwise to make even sized pieces.
In a large bowl, combine carrots with butter, olive oil, salt, and pepper. Transfer carrots to prepared baking sheet and spread into a single layer. Cover baking sheet tightly with foil.
Bake for 15 minutes. Remove foil and continue to roast for 30 minutes, until carrots are well-browned and tender. Use a metal spatula to flip the carrots once or twice during the roasting. Season to taste with additional salt and pepper.