1 1/2 cups milk
1/3 cup all-purpose flour
1/8 teaspoon salt
24 tablespoons (3 sticks) salted butter, softened
1 1/2 cups granulated sugar
2 teaspoons vanilla
In a medium saucepan, add milk, flour and salt - whisk until flour has dissolved. Place over medium heat, whisking, and cook until thickened and beginning to bubble. Reduce heat and cook, stirring, for 2 minutes. Remove from the heat and scoop into a bowl - cover with plastic wrap and place in the refrigerator until it has cooled down.
In a large mixing bowl, cream together butter and sugar until light and fluffy, about 5 minutes. Beat in vanilla extract. Scrape down the sides of the bowl - add the cooled milk mixture, 1/4 cup at a time, beating on low until smooth