To make the mix: Sift together flour, cocoa, soda, and salt; stir to blend; resift with sugar. Label and store in a sealed plastic bag at room temperature. Makes 1 package of mix. Each package makes two 8 or 9 inch layers or one 9x13x2 inch rectangle. To make the cake: Heat oven to 350°F. Grease and flour two 9-inch round baking pans or one 13x9x2-inch baking pan. Cut the shortening into the dry ingredients until the mixture looks crumbly. Add milk, eggs and beat on low speed of electric mixer 30 seconds to blend ingredients. Beat on medium speed 3 minutes, scraping bowl occasionally. Pour batter into prepared pans. Bake 30 to 35 minutes for layers; 35 to 40 minutes for rectangular pan or until wooden pick inserted in center comes out clean. Cool 10 minutes; remove from pans to wire racks. Cool completely. Frost.