Add the butter and one cup of the powdered sugar to the bowl of a standing mixer fitted with the whisk attachment. Beat on low just to blend, then beat on high for 3 minutes, stopping to scrape down the sides of the bowl a few times.
Add the heavy cream and another cup of powdered sugar. Beat on low speed to blend, then on high speed for another two minutes, stopping to scrape down the sides of the bowl a few times.
Add two more cups of powdered sugar and the salt and extracts. Beat on low to combine, then on high for about 1 minute, until the buttercream is completely smooth.
Check the consistency and beat in more powdered sugar if the buttercream is too thin. Taste, and add more salt, vanilla, or almond extract if desired.