Arrange a rack in the middle of the oven and heat the oven to 350ºF. Peel and cut 3 large sweet potatoes into 1-inch pieces.

Place the sweet potatoes in a large saucepan or pot and cover with water by about 1 inch. Bring to a low boil over medium-high heat and simmer just until they can be pierced with a fork and starting to soften (they will not be fully cooked), about 10 minutes. Be careful not to overcook.

Using a slotted spoon, carefully transfer the sweet potatoes into an 8x8-inch square baking dish (reserve the cooking water). Spread them out in an even layer.

Transfer ¾ cup warm reserved cooking water into a measuring cup or medium bowl. Add ½ cup packed dark brown sugar and stir to dissolve. Add ½ teaspoon vanilla extract and stir to combine.

Sprinkle ½ teaspoon ground cinnamon and ¼ teaspoon ground nutmeg over the sweet potatoes.

Pour the water-sugar mixture over the sweet potatoes.

Cut 1 stick salted butter into cubes and scatter over the sweet potatoes.

Bake for 5 minutes. Carefully remove the baking dish from the oven and gently toss the sweet potatoes to combine the sauce and evenly coat them.

Bake until the sweet potatoes are fork-tender, about 10 minutes more. Remove the baking dish from the oven and spoon the candied mixture evenly over the top.

Let the candied yams cool about 5 minutes so the sauce can thicken. Serve warm.