Preheat oven to 400°; grease a 12-cup muffin pan.
In a large bowl, whisk together flour, baking powder, cinnamon, baking soda, and salt.
In a medium bowl, using a potato masher or fork, mash sweet potatoes.
Whisk in brown sugar, egg, buttermilk, marmalade, and oil until well blended.
Add the egg mixture to the flour mixture and stir until just blended.
Divide batter equally among prepared muffin cups.
Bake in preheated oven for 18-22 minutes or until tops are golden and pick comes out clean.
Let cool in pan on a wire rack for 3 minutes, then transfer to the rack to cool while you prepare the icing.
Icing-in a small bowl, whisk together powdered sugar, marmalade, buttermilk, and vanilla until smooth.
Spoon or drizzle over tops of the warm muffins and let cool.