Combine chocolate and milk in top of double boiler and cook over rapidly boiling water 5 minutes, stirring occasionally. Blend with rotary egg beater; cool. Sift flour once, measure, add soda, salt, and sugar and sift together three times. Cream shortening; add flour mixture, vanilla, and chocolate mixture, and stir until all flour is dampened. Then beat vigorously 1 minute. Bake in two greased and lightly floured 8-inch layer pans in moderate oven (375° F.) 20 minutes, or until done.