Using an electric mixer with paddle attachment, beat the unsalted and salted butters on medium speed until smooth and creamy.
Add cream cheese and vanilla.
Beat until fully incorporated.
Gradually increase mixer speed to high and continue beating until light and fluffy, scraping sides of bowl as necessary.
Gradually add 4 cups of confectioners' sugar until desired consistency for spreading is reached, adding more sugar as needed to do so.
Beat on high until well combined and smooth while scraping the sides of the bowl every 1-2 minutes.
Chef's notes: Makes about 4 ½ cups of frosting. Enough for 2 layer 9 inch cake, 3 layer 8 inch cake, 9x13 inch baking pan or 18-24 cupcakes. Set butter out of refrigerator about 30 minutes before use.