Line donut pan with plastic wrap. Transfer Nutella to a piping bag (or zip-lock bag and cut off the corner). Pipe Nutella in the center of each cavity to make the ring, then place in the freezer until completely firm. Remove Nutella rings from the pan, place on a plate and store back in the freezer until ready to assemble donuts.
Preheat the oven to 350F. Brush donut pan with melted butter, then set aside.
In small bowl stir together flour, baking powder and salt.
In another bowl stir together remaining melted butter (3 ½ Tbsp.) and 2 ½ Tbsp. sugar. Add cinnamon, vanilla, egg and milk and whisk just to combine. Whisk in flour mixture.
Transfer the mixture in piping bag (or zip-lock bag and cut off the corner). Fill donut pan with half of the batter. Take the Nutella rings from the freezer and place on top. Cover Nutella with remaining batter.
Bake about 13-15 minutes, or until golden brown and firm to touch.
Coll donuts for 5 minutes in the pan then invert on a rack.
In a bowl combine cinnamon and sugar.
Melt 2 Tbsp. butter.
Brush each donut with melted butter, then dip in cinnamon sugar mixture.