Preheat your oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a large mixing bowl, combine the mashed sweet potatoes, sugar, milk, vegetable oil, and egg. Mix well.
In another bowl, whisk together the flour, baking powder, cinnamon, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Divide the batter evenly among the muffin cups.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.