Cream butter until soft. Add salt and part of sugar gradually, blending after each addition. Then add egg yolks and vanilla; blend well. Add remaining sugar, alternately with milk, until of right consistency to spread, beating after each addition until smooth. Makes 2 1/2 cups or enough frosting to cover tops and sides of two 8 or 9 inch layers.
Note: If desired, 1 whole egg may be substituted for the 2 egg yolks to make 2 3/4 cups frosting.