Ultra-Crispy Roast Potatoes

"[Photographs: J. Kenji Lopez-Alt] Note: For creamier potatoes with a slightly less crisp crust, substitute Yukon Gold potatoes for russets. Duck fat, turkey fat, or chicken fat will give the best results, but bacon fat or olive oil will work well. About the author: J. Kenji Lopez-Alt is the Managing Editor of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook...."