Preheat oven to 400°. Grease a medium baking dish with butter.

In a medium saucepan over medium heat, bring butter, brown sugar, orange juice, bourbon (if using), cinnamon stick, nutmeg, and salt to a boil. Reduce heat to medium-low, bring to a simmer, and cook, stirring occasionally, until slightly thickened, about 10 minutes. Discard cinnamon stick.

Meanwhile, peel yams and slice into ½-inch-thick rounds. Layer in prepared dish. Pour syrup over and cover dish with foil.

Bake yams for 30 minutes, then uncover and continue to bake, basting every 15 minutes, until softened and fork-tender, 50 to 60 minutes more. Let cool slightly.