In a mixing bowl, combine the flour, baking soda, cocoa and salt; set aside. In a separate bowl, combine the sugar, shortening and eggs. Beat for about 2 minutes. Mix the dry ingredients with the egg mixture. Add the milk & warm water and beat at medium speed for 2 to 3 minutes. Add the vanilla and beat again until the ingredients are thoroughly blended. This batter is for a cake like cookie. Bake as drop cookies. Drop by rounded tablespoons full onto ungreased non-stick cookie sheets and bake in a preheated oven at 375° for 10 to 12 minutes or until the center of the cookies spring back when lightly pressed. Remove from the cookie sheets and cool a wire rack or white paper towels.
Beat the egg whites until stiff; set aside. Combine the other ingredients and beat very hard for several minutes at high speed. Fold in the beaten egg whites. To make the whoopie pies - Spread a generous amount of filling on a completely cooled cookie. Top with another cookie. Wrap each whoopie pie individually in plastic wrap.