Cream butter and 3/4 cup sugar. Add yolks, one at a time, beating well after each. Combine orange juice, peel and vanilla. Combine dry ingredients; add to creamed mixture alternately with juice mixture, beating well. In another bowl, beat whites until foamy; gradually add remaining sugar, beating until soft peaks form. Fold into batter. Line two greased 9-in. Round cake pans with waxed paper; grease paper. Pour batter into pans. Bake at 350'F for 20 minutes or until cake tests done. Cool 5 minutes; remove to wire rack. Peel off paper; cool. For filling, combine flour and a small amount of eggnog in a pan; stir until smooth. Stir in remaining eggnog; bring to a boil, stirring constantly. Cook and stir 2 minutes. Cool completely. Cream butter and sugar; add vanilla and nutmeg. Gradually beat in eggnog mixture. For frosting, mix chocolate, sugar, cinnamon and nutmeg. Beat in butter and cream. Add water until frosting drizzles slightly. Split cakes in half; spread filling on three layers. Stack with plain layer on top; frost the top.