Wash yams and boil until tender (test by pricking with a fork). Peel and cut cooked yams into large cubes or slices. Spray a 13"x9" pan with cooking oil and add yams.
Mix sugars, water, corn syrup, and cornstarch in saucepan. Slowly bring to a boil over medium heat. Cook down until mixture thickens. Stir in vanilla and pour over yams. Bake at 375 degrees for approximately 30 minutes, periodically spooning the sauce over the yams.
My grandma passed this recipe on to me before she died. I am assigned to bring Candied Yams to Thanksgiving dinner every year. It's an all around favorite.