Preheat oven to 350; grease donut pan
In a large bowl combine butter and sugar; mix well
To the butter mixture add in flour, baking powder, salt , and cinnamon; stir to combine
Add in milk and mashed sweet potatoes; mix well
To prepare glaze, in a small bowl mix powdered sugar and syrup. Add milk, 1 teaspoon at a time, until glaze is thin
To cook donuts, add batter to each donut cup until approximately half full (tip: put batter in a large plastic storage bag, cut the corner, and pipe into the cups). Cook for 7-8 minutes or until donuts spring back when touched
Allow donuts to cool slightly, remove from pan. While still warm, dip donuts into the glaze
Serve with a cold glass of milk