In a large bowl, cream the butter and shortening. Using an electric mixer, beat on medium speed using the paddle attachment until light and fluffy for about 1 minute. Beat in maple flavoring and scrape down the bowl occasionally.
Gradually add powdered sugar, one cup at a time, starting by mixing each cup on a low speed then beating well on medium speed. Scrape sides and bottom of the bowl after each cup of sugar is added. When all sugar has been mixed in, the frosting will appear dry and stiff.
Gradually add milk or water, adding small amounts at a time until the desired consistency is reached; continue to beat at medium speed until light and fluffy. Scrape down the bowl once more and beat again to make sure all the ingredients are incorporated. Add a pinch of salt if too sweet and mix until completely dissolved.