Preheat oven to 350°.
To prepare cake, coat 2 (8-inch) round cake pans with cooking spray; line bottoms of pans with wax paper. Coat wax paper with cooking spray; set pans aside.
Place granulated sugar and shortening in a large bowl; beat with a mixer at medium speed 5 minutes or until well blended. Add egg substitute; beat well. Combine cocoa and food coloring in a small bowl, stirring with a whisk; add to sugar mixture, stirring well to combine.
Lightly spoon the cake flour into dry measuring cups, and level with a knife. Combine cake flour and 1/2 teaspoon salt, stirring with a whisk. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture; mix after each addition. Stir in 1 teaspoon vanilla. Combine vinegar and baking soda; add to batter, stirring well. Pour batter into prepared pans. Sharply tap pans once on counter to remove air bubbles. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes on a wire rack; remove from pans. Carefully peel off wax paper; cool completely on wire rack.
To prepare frosting, combine milk and all-purpose flour in a small, heavy saucepan over medium heat. Cook 3 minutes or until mixture forms a very thick paste, stirring constantly with a whisk. Spoon into a bowl; cover surface with plastic wrap. Refrigerate 45 minutes or until chilled. Place butter in a medium bowl, and beat with a mixer at medium speed until creamy. Add 2 teaspoons vanilla, 1/8 teaspoon salt, and chilled flour mixture; beat until smooth. Gradually add powdered sugar, beating just until blended (do not overbeat).
Place 1 cake layer on a plate; spread with 1/3 cup frosting. Top with other cake layer. Spread remaining frosting over top and sides of cake. Store cake loosely covered in refrigerator.