Beat the butter, shortening, and vanilla on medium-high speed until smooth (scrape down the sides as necessary).
Beat on low speed as you add the confectioners erythritol and casein. Then on high until light and fluffy (a few minutes).
Notes
Store the frosting in the refrigerator in an air-tight container and bring to room temperature before using. You may want to mix it again for a couple of minutes before frosting your cake.