In a large bowl, beat together butter, shortening & extracts until smooth and creamy.
Add 2 tablespoons of the milk or water, the salt, and half the powdered sugar & mix until just combined.
On low speed, gradually add the remaining powdered sugar. Add more milk or water as needed.
If using within 2 hours, the frosting can be left at room temperature. Just keep the frosting covered with a damp cloth to prevent crusting.
If NOT using in 2 hours, store the frosting in the refrigerator in an airtight container. When ready to use, bring the frosting to room temperature and rewhip to soften before using.