For base: Cut shortening into dry ingredients; mix well. Pat firmly and evenly into an 8-inch square pan. Bake at 350F for 10-12 min. Cool. For cake: Cream shortening and sugar; add eggs and vanilla, beating well until fluffy. Sift flour, baking powder and salt together; add to creamed mixture alternately with milk. Pour batter into a g greased and floured 8-inch square pan. Bake at 350F for 35-40 min. Cool. For frosting: In small saucepan, stir together the milk and the flour. Cook, stirring constantly until mixture is thickened and smooth; cool. On highest speed of mixer, beat cooled flour mixture wit h butter, shortening and vanilla until smooth and fluffy. Blend in icing sugar and salt; continue beating until frosting is fluffyat least 15 min. Assemble. To Assemble Cake: Place shortbread base on serving plate; spread with small amount of frosting. Place cake on base. Cover top and sides of cake with remaining frosting. If desired, garnish with chocolate sprinkles or decorate as you wish.