Blend 1/2 cup milk and 1 tablespoon cornstarch. Cook and stir over low heat until mixture thickens and boils. Cool. Cream 1/2 cup shortening and 1 cup sifted confectioner's sugar until fluffy. Beat in cornstarch mixture, 1/2 teaspoon salt, and 1 1/2 teaspoons vanilla. Add 1 1/2 cups sifted confectioner's sugar, beating smooth. Frosts tops and sides of two 8-9 inch layers.