Preheat oven to 350oF (180oC). Line a standard 6 cups muffins pan with paper liners. Set aside. To a bowl, add cooked sweet potato, sugar, oil, water and lemon juice (or vinegar). Whisk until sugar is completely dissolved.
Sift in flour, baking powder, baking soda and cinnamon powder.
Mix until dry flour is no longer visible. DO NOT OVER MIX (mix until dry and wet ingredients are just combined).
Spoon batter into cupcake liner until 2/3 full.
Sprinkle top of cupcakes with sugar.
Bake in preheated oven to 350oF (180oC), rack middle position, 18-20 minutes, or until toothpick inserted in the middle of cake comes out clean.
Let cupcakes cool down in pan for 10 minutes. Then transfer cupcakes to wire rack to cool down completely.