Add orange and lemon rind to fruit juice and let stand 10 niinutes; strain if desired. Cream butter; add egg yolk and salt and mix well. Add part of sugar gradually blending after each addition. Add remaining sugar, alternately with fruit juice, until of right consistency to spread. Beat after each addition until smooth. Makes enough frosting to cover tops and sides of two 8- or 9-inch layers, or top and sides of 13x9x2-inch cake.