Prep the yams. Preheat your oven to 350ºF and place a rack in the middle position. Peel the 3 large yams and cut them into 1-inch pieces.

Partially cook the yams. In a large saucepan or pot, cover the yams with water, ensuring there’s about 1 inch of water above them. Bring to a gentle boil over medium-high heat. Simmer until the yams are slightly tender and can be pierced with a fork, about 10 minutes. Be cautious not to overcook them.

Transfer to a baking dish. Using a slotted spoon, carefully move the yams to an 8x8-inch square baking dish, arranging them in an even layer. Reserve the cooking water for later use.

Combine the water, brown sugar, and vanilla. Measure out 3/4 cup of the warm reserved cooking water and pour it into a measuring cup or medium bowl. Add the 1/2 cup packed dark brown sugar, stirring until dissolved. Mix in the 1/2 teaspoon of vanilla extract.

Top the yams with spices. Evenly sprinkle 1/2 teaspoon ground cinnamon and 1/4 teaspoon ground nutmeg over the yams.

Pour the sugar mixture over yams. Carefully pour the water-sugar mixture over the prepared yams in the baking dish.

Top with butter. Cut the stick of salted butter into cubes and scatter them over the yams.

Bake 5 minutes. Place the baking dish in the oven and bake for 5 minutes. Remove it from the oven and gently toss the yams to coat them evenly with the sauce.

Continue baking until fork tender. Return the dish to the oven and bake until the yams are fork-tender, about 10 more minutes. Take the dish out of the oven and spoon the candied mixture evenly over the top.

Serve. Allow the yams to cool for about 5 minutes to let the sauce thicken slightly. Serve warm.