Sift together flour, baking soda, cinnamon and salt; set aside. Cream together the shortening and sugar in a mixing bowl until light and fluffy, using an electric mixer at medium speed. Add eggs, one at a time, beating well after each addition. Blend in vanilla and applesauce. Add dry ingredients alternately with buttermilk to creamed mixture, beating well after each addition. Beat in boiling water. (Batter should be thick.)
Pour batter into greased and waxed paper-lined 13 x 9 x 2-inch baking pan. Bake in preheated 350 degree F. oven for 1 hour or until cake tests done. Cool 10 minutes in pan on rack. Remove from pan; cool on rack. When cake is cooled, spread with Chocolate Fluff Frosting. Cut in squares.
CHOCOLATE FLUFF FROSTING: Melt chocolate over hot water. Cool to room temperature. Blend together confectioners' sugar, butter, melted chocolate and vanilla in a bowl and beat until smooth. In another bowl, beat egg whites until soft peaks form, using an electric mixer at high speed. Gradually beat in 1 c sifted confectioners' sugar, 2 T at a time, until egg white mixture is glossy and stiff. Fold in chocolate mixture into the beaten egg whites.