The day before making sweet potato muffins, cook the sweet potatoes and puree them.
Preheat the oven to 325F.
Mix together the flour, cinnamon, baking soda, baking powder, and salt in a large bowl. In another mixing bowl, add the sugar, oil, eggs, and vanilla and mix well. Mix together the dry and wet ingredients. Finally, add the sweet potato and mix well.
Fill the muffin cups (preferably silicon muffin cups) with the muffin batter. Sprinkle flaxseed over the top of each muffin, if desired.
Bake muffins for 32-35 minutes.
Let cool. If freezing, put muffins into a gallon size zip top bag and put in the freezer for up to 3 months.
Adapted from Good in the Simple