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.