1. Combine the butter or margarine, brown sugar, salt and milk or cream in the upper part of a double boiler. Heat over hot water, stirring constantly until smooth. Remove from the heat and let the mixture cool slightly.
2. Gradually add the sugar, a little at a time, beating well after each addition. Use only enough sugar to make an icing of the proper consistency to spread.
3. Beat in the vanilla.Frosting or icing for top of 2 9-inch layers.