Heat oven to 450 degrees F and arrange rack in the middle.
While oven heats, place pecans on a baking sheet to toast until they smell nutty and are slightly darker, about 5 minutes. Remove from oven and set aside.
Place sweet potatoes on a baking sheet, drizzle with oil. Season well with salt and freshly ground black pepper, and toss to coat. Roast until golden brown and tender when pierced with a knife, about 40 to 45 minutes.
Meanwhile, melt butter in a small saucepan over medium heat. When it foams, stir in maple syrup and cayenne and let cook briefly, about 1 minute. Serve sweet potatoes with a drizzle of maple butter and a scattering of pecans.