Source  Mel's Kitchen Cafe

Recipe Added:  11/21/2011

In a medium bowl, combine sugar, flour, cornstarch and salt. Whisk in milk until mixture is smooth. Place a  strainer over a medium saucepan and pour milk mixture through strainer into saucepan. Cook mixture over medium heat, whisking constantly, until mixture boils and thickens, 5-10 minutes. Transfer mixture to a clean bowl and cool to soften.

When cooled, beat mixture with vanilla on low speed until well combined, about 30 seconds. Add butter, one piece at a time, and beat until all butter has been incorporated, about 2 minutes. Increase speed to medium-high and beat 5 minutes, until light and fluffy. Let frosting sit softened until stiff, about 1 hour.

Chocolate Frosting  Add 1/4 cup cocoa powder to sugar, flour, cornstarch, salt mixture in the first step. Once the frosting has been mixed and is light and fluffy, add 3 oz semisweet chocolate, melted and cooled.

1. http://www.melskitchencafe.com/2010/09/the-best-frosting-ever.html