I found this on the web, I have one like it somewhere but can't locate it yet. It wasn't called Custard Frosting as I remember, if it is different when I find it I will post it. This one sounds close to what I remember tho.

CUSTARD FROSTING
This old fashioned frosting is made with a milk and flour mixture beat into sugar and butter mixture.

2 tablespoons flour
3/4 cup milk
3/4 cup butter
3/4 cup sugar
1/8 teaspoon salt
1 teaspoon vanilla

In a saucepan add a small amount of the milk to flour then stir to make a smooth paste. Add remaining milk. Cook, stirring constantly, over medium heat until mixture boils and thickens. Cool.

Cream butter using medium speed of electric mixer. Gradually add sugar and salt; beat well. Add cooled milk mixture. Beat until light and fluffy; add vanilla.

Frosts a two layer cake.