Preheat oven to 325 degrees F and grease the sides and bottom of a mini loaf pan with cooking spray.
First, mix the dry ingredients. In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt until combined. Set aside.
Next, mix the wet ingredients. In a large bowl, cream together softened butter and sugar, then beat in the eggs and vanilla extract. Finally, add the mashed sweet potato and stir thoroughly until fully incorporated and the consistency is smooth.
Fold the dry ingredients into the wet ingredients until combined.
Spoon the batter into the mini loaf pan (it will be thick!), filling the wells about halfway. Smooth out the top and sprinkle with extra sugar.
Bake for 35 to 45 minutes, or until an inserted toothpick comes out clean.
Let loaves cool completely before removing from the pan and slicing or storing.