Peel and slice the 4 pounds yams/garnet sweet potatoes into 1/2-inch rounds, making sure they are evenly sliced for even baking, and set aside.
In a large bowl, mix together the melted ½ cup salted butter, ¾ cup light brown sugar, ½ cup granulated sugar, 2 tablespoons honey, ½ teaspoon ground cinnamon, ¼ teaspoon ground allspice, ¼ teaspoon ground nutmeg, and 1 teaspoon vanilla extract. Add in the sliced yams/sweet potatoes and toss to coat. Transfer the potatoes and any additional sugar mixture into a 9x13-inch baking dish.
Cover the baking dish with aluminum foil and bake for 30 minutes. Remove from the oven and peel back the foil and stir the potatoes. Place the foil back over the top of the pan and bake for an additional 15 to 20 minutes or until yams are fork-tender.
Remove from the oven and allow them to sit for 10 to 15 minutes before serving.
Notes
With Marshmallows: Once the yams are done cooking, sprinkle mini marshmallows over the top of them and then put under the broiler for a couple of minutes until the marshmallows are golden brown.