Heat your oven to 400 degrees Fahrenheit. Lightly grease the bottom of a 9x13 inch baking dish with a small amount of butter or nonstick spray.

Peel the sweet potatoes and slice each one in half lengthwise so you have flat cut sides. Pat them dry with a paper towel so the glaze has something to stick to.

In a small saucepan or microwave-safe bowl, melt the butter. Stir in the brown sugar, cinnamon, nutmeg, salt, cayenne if using, and water until the sugar is mostly dissolved and the mixture is pourable.

Arrange the sweet potato halves cut-side down in a single layer in the baking dish. Pour the brown sugar and butter mixture evenly over the top of all the potatoes, letting it pool around the sides and bottom of the dish.

Roast uncovered for 20 minutes. At the 20-minute mark, open the oven and carefully spoon or brush the glaze from the bottom of the pan back up over the potatoes. This is your one basting step, and it makes a real difference in color and flavor.

Return the pan to the oven and roast for another 18 to 20 minutes, until the cut sides are deeply golden and caramelized and a fork slides into the thickest part with no resistance.

Remove from the oven and let the potatoes rest in the pan for 5 minutes before transferring to a serving platter. Spoon any remaining glaze from the pan over the top before serving.