1

In a medium saucepan, whisk together milk, flour and salt. Cook and stir over medium heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes more. Transfer to a small bowl; cover surface with plastic wrap. Refrigerate until cooled (do not stir).

2

In a large bowl, beat butter, sugar and vanilla on medium speed for 5 minutes until light and fluffy and sugar is almost dissolved. Add cooled milk mixture, ¼ cup at a time; beat on low after each until smooth.