Preheat oven to 375 degrees. Cut the sweet potatoes into 1⁄2-inch slices. Steam the sweet potatoes for 10 minutes or until soft but not mushy. Remove potatoes from the steamer and set aside.

In a small saucepan, melt the margarine over medium heat. Add the brown sugar, vanilla, and salt, stirring until the sugar is dissolved. Stir in the whiskey or bourbon and cook for 5-10 minutes or until slightly thickened, stirring occasionally.

Spray a large 9- by 13-inch glass baking dish with nonstick cooking spray. Place the sweet potatoes in the pan and drizzle the sauce over the top.

Bake uncovered for 1 hour. Baste with the sauce every 10 minutes to keep the potatoes from drying out.