This section is from the book "Queen's Of Cuisine", by Martha Logan. Also available from Amazon: Queens of Cuisine.
Filling and frosting for three 8-inch layers or two 9-inch layers.
1/2 cup Swift'ning 3/4 cup sifted cocoa 1 egg yolk 1 3/4 cups sifted confectioner's sugar.
1 teaspoon vanilla 1/4 cup boiling water teaspoon salt.
Cream Swift'ning until soft and fluffy. Add egg yolk, vanilla, and salt. Beat thoroughly. Sift together cocoa and confectioner's sugar. Add alternately with boiling water. Beat until smooth and creamy.