In a mixing bowl, cream butter and brown sugar. Add eggs, one at a time, beating well after each addition. In a bowl, combine the syrup, milk, sour cream and maple flavoring.
In another bowl, combine the flour, baking powder, baking soda, salt, clovs, allspice, nutmeg and mace; add to creamed mixture alternately with syrup mixture.
Pour into two greased and floured 9 inch round baking pans. Bake at 350 for 20-25 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to a wire rack to cool.
For frosting, in a mixing bowl, cream butter and maple flavoring. Gradually beat in the confectioners' sugar. Add enough sour cream to acheive spreading consistency. Spread frosting between layers and over top and sides of cake. Sprinkle with walnuts if desired. Store in the refrigerator.