With an electric mixer, blend the butter and Nutella until smooth and well combined. With the mixer on low speed, add the powdered sugar 1/4 cup at a time, mixing well after each addition. Be sure to scrape down the sides of the bowl a couple times. Add 1-3 teaspoons of milk, one teaspoon at a time, to reach the desired consistency for piping or filling and frosting. Whip on high speed until light and fluffy. Yields approximately 3-1/2 cups icing.