Prep the pan and spices. Butter a 13 x 9-inch dish. Peel the yams, slice them into rounds, then cut each round into quarters so the pieces are medium, not small. Stir the cinnamon, ginger, allspice, nutmeg and cloves together and set aside.
Brown the butter. Melt the 2 sticks of butter in a saucepan and keep cooking until it turns nutty brown and smells toasty.
Build the syrup. Add the brown sugar to the brown butter and stir; it will be thick. Stir in the orange juice and maple syrup, then pull the pan off the heat and stir in the spice mix. If you are using cognac, stir it in here, off the heat.
Sauce the yams. Put the yams in the buttered dish. Pour the hot syrup over them and toss to coat.
Let it sit for 45 minutes. Leave the yams in the syrup so the sauce thickens and the spice works in. Do not skip this step.
Coat again. Toss the yams in the syrup once more. It should be thick and glossy, with a layer of syrup on the bottom of the dish and every piece fully coated.
Bake covered. Cover and bake at 350 degrees F for 45 to 50 minutes, until a fork slides through. They should be soft but still holding their shape.
Marshmallow it. Remove from the oven and cover the top with a full, generous layer of regular-size marshmallows.
Toast. Return to the oven at 400 degrees F until the marshmallows puff up and are nicely toasted. Watch closely, this step moves fast.