Heat the oven to 400 degrees Fahrenheit. Allow the oven to fully preheat so the sweet potatoes begin cooking evenly as soon as the dish goes in.

Peel and rinse the sweet potatoes, then slice them into rounds between ½ inch and 1 inch thick. Cut larger rounds in half if needed so the pieces fit comfortably in the baking dish.

Butter a small baking dish and arrange the sweet potato rounds inside. Keep the slices in an even layer so the syrup can reach each piece.

Melt the butter in a small saucepan over medium heat, then lower the heat and stir in both sugars, cinnamon, nutmeg, and vanilla for about 3 minutes. The mixture should look smooth and syrupy, with a warm buttery cinnamon scent.

Pour the warm syrup over the sweet potatoes and cover the baking dish tightly with foil. The foil traps steam, helping the thick-cut rounds soften while they absorb the spiced glaze.

Bake for 45 minutes, then remove the foil, gently turn the sweet potatoes in the syrup, raise the oven temperature to 425 degrees Fahrenheit, and bake for 15 minutes more. The sauce should bubble around the edges and deepen to a glossy amber color as it reduces.

Remove the dish from the oven and let it cool briefly before serving. A fork should slide into a round with gentle resistance, while the syrup should coat the back of a spoon.