Heat oven to 400 degrees F. Prick the potatoes with a fork and place on a microwave-safe plate. Cook on high for 6 minutes; turn each potato and cook until soft, 6 minutes more. Let stand until cool enough to handle.
Meanwhile, on a rimmed baking sheet, toss the pecans with the oil, then the Parmesan, cayenne, 2 teaspoons rosemary, and ½ teaspoon salt. Roast for 8 minutes. Toss with the remaining teaspoon rosemary.
Peel the potatoes and mash in a bowl with the maple syrup and ½ teaspoon salt until smooth. Spoon into a shallow 2-quart baking dish and top with the pecan mixture. Bake until heated through, 6 to 8 minutes.
