Cook sweet potatoes until very soft. (See How to cook Sweet potato in the microwave.)
Scoop out the flesh to fill 2 cups and mash with a fork or potato masher. Set aside.
Coarsely chop pecans.
Preheat oven to 180°C, brush 20 to 24 muffin cups with butter or oil, or line with paper liners.
Sift and mix together, flour, salt, baking powder, baking soda, cinnamon, ginger and nutmeg, in a bowl.
In another bowl, mix together eggs, vegetable oil, butter.
Add sugar, brown sugar, chopped pecan or cashew nuts (leave some for decoration) and sweet potato.
Add the flour mixture to the sweet potato mixture and stir to just combine.
Fill muffin cups up to three-quarter full.
Sprinkle reserved nuts on top of muffins.
Bake for 25 minutes, until a skewer inserted into the middle, comes out clean.
Cool in cups for 5 minutes then remove and cool completely on a wire rack.
Baking Sweet potato muffins mix in a Loaf pan: