Preheat oven to 350°F. Line muffin pans with paper liners.
To steam the sweet potatoes, peel them and cut into approximately 1-inch chunks. Place the chunks into a shallow bowl (I used a large dinner plate with a rim) and pour 3 tablespoons of water over them. Cover the bowl or plate with plastic wrap and microwave on high for about 15 minutes. You should be able to poke a fork through a chunk easily. Pour off the water and mash sweet potato chunks with a fork. Retain 1½ cups of mashed sweet potatoes for use in the recipe. Allow them to cool while you combine the other ingredients.
In a large mixing bowl, whisk together flour, granulated sugar, brown sugar, baking soda, five-spice powder, and salt. Set aside.
In another large mixing bowl, whisk together mashed sweet potatoes, eggs, oil, maple syrup, sour cream, and vanilla extract until smooth. Pour wet ingredients into dry ingredients and stir together to combine. Do not overmix. Try to stop stirring just before all of the flour mixture disappears.