Boiling Heat oven to 350 degrees. Sift flour, sugar, cocoa, baking soda and salt into a 9x13-inch baking pan. Add water, oil, vinegar and vanilla. Mix with a large rubber spatula until smooth, taking care to mix ingredients along edges. Bake until toothpick inserted in center comes out clean, about 40 minutes. Cool completely on wire rack. FROSTING: combine butter and shortening in a food processor or mixing bowl. Add sugar and vanilla. Mix until smooth, about 2 minutes. Slowly drizzle in milk. Beat 2 minutes in food processor or 7-10 minutes with a mixer, until smooth, satiny and fluffy. Chill just 20 minutes before frosting. Helen Jolly ----