In a small bowl, cream shortening and sugar untill fluffy. Add egg yolk; beat well. Stir in vanilla. Combine the flour, cocoa, salt, baking powder, baking soda, cinnamon, and nutmeg in a separate bowl and in another bowl combine milk and vinegar. Alternately add flour mixture and milk mixture to butter mixture until well combined. Fold in walnuts.
In a small bowl beat egg whites to form soft peaks and gradually fold egg whites into batter. Transfer to 2 greased 6-inch backing pans. Bake at 350 F for 15-20 minutes Cool for 10 minutes before removing from pans.
For frosting, in a small saucepan, combine flour and milk until smooth. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Cover; cool to room temperature In another small bowl, cream buter and shortening. Gradually beat in sugar. Gradually add cooled milk mixture and beat 4 minutes or until fluffy. Add vanill and salt, beat well. Spead between layers and evenly on top.