Melt butter in top part of double boiler.
Add sugar and cook for 2 minutes over direct heat.
Add 1 cup milk and heat over boiling water.
Mix flour, remaining milk, and salt until smooth.
Gradually stir into hot mixture.
Cook for 5 minutes, stirring constantly.
Cover; cook for 10 minutes longer, stirring occasionally.
Add mixture gradually to egg; return to double boiler and cook for 2 minutes.
Cool; add vanilla.
Use as filling in layer cake.
Makes about 2 cups.