CHOCOLATE WHOOPIE PIES

½ cup shortening                           1 cup milk
1 cup sugar                                         2 cups flour
3 Tbsp cocoa                                    1 tsp baking powder
2 egg yolks                                         ½ tsp salt
1 tsp vanilla                                        1 tsp soda

¼ cup butter                                    1 ¼ cups icing sugar
¼ cup shortening                          2 egg whites beaten