Preheat oven to 350 F and prepare mini donut pan. Spray with oil.
In a small bowl mix flour, baking powder, baking soda and cinnamon. Set aside.
In a medium bowl mix egg, egg yolk, sweet potato, melted butter, oil, buttermilk and vanilla extract. Or you can also blend in the blender so the mixture will be more smooth.
Add sugar and mix well with a spoon.
Add dry ingredients and mix with spoon or spatula until all ingredients are incorporated well.
Put the batter into the bag, make a small hole on the corner. Push gently the batter into the baking pan and form mini donuts.
Bake for 10 - 13 minutes.
Melt butter in the saucepan, add brown sugar and cook on low heat for 2 minutes. Add milk and cook for few minutes on low heat. Mixing all the time. Add confectioners sugar and mix all the time. Cook on low heat for 1 -2 minute. Check the icing, if it's to grainy add a little milk, or if it's to watery add more confectioners sugar. Whisk until all is dissolved well.