Preheat oven to 400 degrees. Spray an 8-inch square baking dish with cooking spray and set aside. Melt 2 teaspoons butter in a cup in the microwave.
Bake yams until soft, 40 to 60 minutes, depending on size. Peel yams as soon as they are cool enough to handle. Place flesh in a food processor or large bowl.
Add remaining 2 tablespoons butter and maple syrup to hot yams. Process to purée, or mash with a fork, until yams are smooth. Season to taste with salt and pepper. Spread yams into prepared baking dish, making an even layer.
Peel, halve and core apple. Place each half cut-side down on a cutting board and cut it crosswise into thin slices. Arrange slices in overlapping rows to cover yams. Brush apples lightly with the melted butter.
Bake uncovered until yams are heated through and apples have softened, 25 to 30 minutes. Serve warm or at room temperature.
Makes 8 1/2-cup servings, each containing 267 calories and 4 grams of fat.
