Creamy white filling

1/3 cup plus 3 tbl. sifted flour
 1 1/2 cups milk
 3/4 pound (3 sticks) butter
 Generous pinch of salt
 1 1/2 tsp. vanilla extract
 3 3/4 cups strained powdered sugar
 Place the flour in a 1-quart saucepan.  Add the milk gradually,
 stirring with a rubber spatula.  If the mixture is not smooth, strain
 it before cooking.  Place it over moderate heat.  Cook, stirring and
 scraping the bottom constantly with a rubber spatula, until the
 mixture becomes very thick and bubbles slightly.  Simmer, stirring,
 for about 2 minutes.  If necessary, beat with a small wire whisk to
 make the mixture smooth.Stir in 1 tbl. of the butter and set the
 mixture aside to cool to room temperature - stir it occasionally while
 it is cooling.  Place the remaining butter in the large bowl of an
 electric mixer and beat until it is slightly softened.  Add the salt,
 vanilla, and sugar (gradually) and beat well for about 2 minutes,
 scraping the bowl as necessary with a rubber spatula.  Gradually, 1
 large spoonful at a time, add the cooled flour and milk mixture.  Then
 beat at high speed for a minute or two until the filling is smooth,
 light, and fluffy.