While the oven is preheating, use 1 tablespoon of butter to grease a 9x13-inch baking dish.
Peel the yams, rinse, and slice into ¼-inch slices.
Place the slices into the baking dish.
In a small saucepan on the stove, heat the remaining butter over medium heat until melted. Then, remove from the heat.
Wash and zest the orange, and add to the melted butter. Add the molasses, light brown sugar, salt, cinnamon, and ginger, and stir until well combined.
Pour the mixture over the yams, and use a spatula to turn the yams to evenly distribute the butter mixture. Cover with foil, and bake for 45 minutes.
Remove the foil, stir again (bringing the yams from the bottom to the top), and bake for another 15 to 20 minutes, until the yams are tender and a paring knife slides in and out easily.