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