Peel the sweet potatoes and slice about 1 ¼ inches thick. Add ½ teaspoon salt to water in a saucepan and bring to a boil. Add potato slices and simmer for about 12 minutes, or until somewhat softened. Remove carefully to a wire rack and allow to cool for about 10 minutes.

Line a large, heavy sheet pan with parchment paper. Using a flat-bottomed bowl, slightly mash each potato slice, taking care that they stay intact. Drizzle each slice with a mixture of the butter and oil, using half of the mixture.

Blend the topping ingredients (sugar and all the spices) together really well with a fork, making to sure to get the sugar completely incorporated.

Sprinkle the slice evenly with a total of half the topping ingredients. Make sure you avoid getting the topping on the sheet between the slices if possible—it will burn and smoke before the potatoes are done.

Gently turn over each slice, drizzle with the remaining butter/oil mixture and sprinkle with the remaining seasonings.

Roast at 375° for 20 minutes, carefully flip each slice and return to oven for about 10 minutes more. Serve hot.